Subnetting: dividing a local network using vlan

it could be used to resolve addresses on different networks. In fact, ARP can be used with arbitrary physical addresses and network protocols. The ARP protocol assumes that each device knows both its IP address and its physical address. ARP dynamically links them and enters them into a special table where IP-address pairs are stored. res – physical address(usually each entry in the ARP table has a lifetime of 10 minutes). This table is stored in the computer's memory and is called a cache. ARP protocol(ARP cache).

ARP works by sending messages between network nodes:

ARP Request is a broadcast request sent at the physical layer of the TCP/IP model to determine MAC addresses of a host that has a specific IP address;

ARP Reply (ARP response) - node, The IP address of which is contained in the ARP request sends information about its MAC address to the node that sent the ARP request;

RARP Request, or Reverse ARP Request

ARP request) - a request to determine an IP address using a known MAC address;

RARP Reply, or Reverse ARP Reply

response) - the host's response to a reverse ARP request.

9.1.3.1. Subnetting networks using a subnet mask

For more effective use IP network address spaces can be divided into smaller subnets (subnetting) or combined into larger ones using a subnet mask.

supernetting.

Let's take an example of dividing the 192.168.1.0/24 network (class C network) into smaller subnets.

In the source network, the IP address has 24 bits for the network ID and 8 bits for the host ID. We use a 27-bit subnet mask, or, in decimal notation, - 255.255.255.224, in binary notation - 11111111 11111111 11111111 11100000. The resulting partition is shown in Table 9.3.

		Table 9.3
Dividing the 192.168.1.0/24 network into subnets

	IP range	Broadcast
		subnet address
		subnet address








192.168.1.128/27	192.168.1.129–

192.168.1.160/27	192.168.1.161–

192.168.1.192/27	192.168.1.193–

192.168.1.224/27	192.168.1.225–

Thus, we got 8 subnets, each of which can have up to 30 nodes. Recall that a host ID consisting of all zeros denotes the entire subnet, and a host ID consisting of all ones denotes a broadcast address (a packet sent to such an address will be delivered to all nodes on the subnet).

IP addresses in these subnets will have the structure shown in Figure 9.5.

Rice. 9.5. Structure of IP addresses in network subnets 192.168.1.0/24

Similarly, you can use a subnet mask to combine small networks into larger ones.

For example, the IP addresses of the network 192.168.0.0/21 will have the following structure, shown in Figure 9.6.

Rice. 9.6. Structure of IP addresses of the network 192.168.0.0/21

The IP address range of this network is: 192.168.0.1–192.168.7.254 (2046 nodes in total), subnet broadcast address is 192.168.7.255.

Benefits of subnetting internally private network:

splitting large IP networks on a subnet (subnetting) allows you to reduce the amount of broadcast traffic (routers do not allow broadcast packets);

combining small networks into larger networks (supernetting) allows you to increase the address space using lower-class networks;

changing the topology of a private network does not affect routing tables on the Internet (they only store a route with a common network number);

the size of global routing tables on the Internet does not grow;

the administrator can create new subnets without having to obtain new network numbers.

The most significant bits of the IP address are used by workstations -

mi and routers to determine the address class. Once the class is determined, the device can uniquely calculate the boundary between the bits used to identify the network number and the bits of the device number on that network. However, when dividing networks into subnets or when combining networks to determine the boundaries of bits identifying the subnet number, this scheme is not suitable. For this purpose, a 32-bit subnet mask is used, which helps to unambiguously determine the required boundary. Recall that for standard network classes, masks have the following meanings.

To more efficiently use the address space, IP networks can be divided into smaller subnets (subnetting) or combined into larger networks (supernetting) using a subnet mask.

Let's take an example of dividing the 192.168.1.0/24 network (class C network) into smaller subnets. In the source network, the IP address has 24 bits for the network ID and 8 bits for the host ID. We use a 27-bit subnet mask, or, in decimal notation, - 255.255.255.224, in binary notation - 11111111 11111111 11111111 11100000. We get the following subnetting:

Table 4.3.
Subnet	IP address range	Broadcast address in the subnet
192.168.1.0/27	192.168.1.1–192.168.1.30	192.168.1.31
192.168.1.32/27	192.168.1.33–192.168.1.62	192.168.1.63
192.168.1.64/27	192.168.1.65–192.168.1.94	192.168.1.95
192.168.1.96/27	192.168.1.97–192.168.1.126	192.168.1.127
192.168.1.128/27	192.168.1.129–192.168.1.158	192.168.1.159
192.168.1.160/27	192.168.1.161–192.168.1.190	192.168.1.191
192.168.1.192/27	192.168.1.193–192.168.1.222	192.168.1.223
192.168.1.224/27	192.168.1.225–192.168.1.254	192.168.1.255

IP addresses in these subnets will have the structure:

We note very important point. Using such a mask, nodes with, for example, IP addresses such as 192.168.1.48 and 192.168.1.72 are located in different subnets, and for the interaction of these nodes, routers are needed that forward packets between the subnets 192.168.1.32/27 and 192.168.1.64/ 27.

Note. According to TCP/IP protocol standards, for this example, subnets 192.168.1.0/27 and 192.168.1.224/27 (that is, the first and last subnets) should not exist. In practice, most operating systems (including systems from the Microsoft Windows) and routers support such networks.

Similarly, you can use a subnet mask to combine small networks into larger ones.

For example, the IP addresses of the network 192.168.0.0/21 will have the following structure:

The IP address range of this network is: 192.168.0.1–192.168.7.254 (2046 nodes in total), subnet broadcast address is 192.168.7.255.

Benefits of subnetting within a private network:

dividing large IP networks into subnetting allows you to reduce the amount of broadcast traffic (routers do not allow broadcast packets);
combining small networks into larger networks (supernetting) allows you to increase the address space using lower-class networks;
changing the topology of a private network does not affect routing tables on the Internet (they only store a route with a common network number);
the size of global routing tables on the Internet does not grow;
the administrator can create new subnets without having to obtain new network numbers.

The most significant bits of an IP address are used by workstations and routers to determine the class of the address. Once the class is determined, the device can uniquely calculate the boundary between the bits used to identify the network number and the bits of the device number on that network. However, when dividing networks into subnets or when combining networks to determine the boundaries of bits identifying the subnet number, this scheme is not suitable. For this purpose, a 32-bit subnet mask is used, which helps to unambiguously determine the required boundary. Recall that for standard network classes masks have the following meanings:

255.0.0.0 – mask for class A network;
255.255.0.0 - mask for class B network;
255.255.255.0 - mask for class C network.

It is extremely important for a network administrator to have clear answers to the following questions:

How many subnets does an organization need today?
How many subnets might an organization need in the future?
How many devices are in the organization's largest subnet today?
How many devices will be on an organization's largest subnet in the future?

Avoiding using only the standard IP network classes (A, B, and C) is called Classless Inter-Domain Routing. CIDR).

Introduction to IP Routing

First, let's clarify some concepts:

network node (node) - any network device with TCP/IP protocol;
host (host) - a network node that does not have packet routing capabilities;
router - a network node with packet routing capabilities

IP routing is a forwarding process unicast-traffic from the sending node to the recipient node in an IP network with an arbitrary topology.

When one node on an IP network sends a packet to another node, the IP packet header contains the IP address of the sending node and the IP address of the receiving node. The packet is sent as follows:

The sending node determines whether the receiving node is on the same IP network as the sender (in local network), or on another IP network (on a remote network). To do this, the sending node performs a bitwise logical multiplication of its IP address by its subnet mask, then a bitwise logical multiplication of the IP address of the recipient node also by its subnet mask. If the results match, then both nodes are on the same subnet. If the results are different, then the nodes are on different subnets.
If both network nodes are located on the same IP network, then the sending node first checks the ARP cache to see if the MAC address of the recipient node is in the ARP table. If the required entry is available in the table, then the packets are then sent directly to the recipient node at the link level. If the required entry is not in the ARP table, then the sending node sends an ARP request for the IP address of the recipient host, the response is placed in the ARP table and after that the packet is also transmitted at the data link level (between network adapters of computers).
If the sending node and the receiving node are located on different IP networks, then the sending node sends Current Package to the network node, which is specified in the sender's configuration as the "Primary Gateway" ( default gateway). The default gateway is always on the same IP network as the sending node, so communication occurs at the data link layer (after an ARP request is made). The default gateway is the router that is responsible for sending packets to other subnets (either directly or through other routers).

Consider the example shown in rice. 4.5.

Rice. 4.5.

IN in this example 2 subnets: 192.168.0.0/24 and 192.168.1.0/24. The subnets are combined into one network by a router. The router interface in the first subnet has an IP address of 192.168.0.1, in the second subnet - 192.168.1.1. The first subnet has 2 nodes: node A (192.168.0.5) and node B (192.168.0.7). The second subnet has host C with the IP address 192.168.1.10.

If Host A sends a packet to Host B, it will first figure out that Host B is on the same subnet as Host A (ie, the local subnet), then Host A will make an ARP request for IP address 192.168. 0.7. After this, the contents of the IP packet will be transmitted to link layer, and the information will be transferred from the network adapter of host A to the network adapter of host B. This is an example of direct data delivery (or direct routing, direct delivery).

If host A sends a packet to host C, it will first figure out that host C is on a different subnet (i.e., a remote subnet). After this, node A will send a packet to the node that is specified in its configuration as the default gateway (in in this case this is the router interface with IP address 192.168.0.1). The router on interface 192.168.1.1 will then make a direct delivery to Host C. This is an example of indirect delivery (or indirect delivery) of a packet from Host A to Host C. In this case, the indirect routing process consists of two direct routing operations.

In general, the IP routing process is a series of individual operations that directly or indirectly route packets.

Each network node makes a decision about packet routing based on the routing table, which is stored in random access memory of this node. Routing tables exist not only for routers with multiple interfaces, but also for workstations connected to the network via a network adapter. The routing table in Windows can be viewed using the route print command. Each routing table contains a set of entries. Records can be generated in various ways:

entries created automatically by the system based on the TCP/IP protocol configuration on each network adapter;
static entries created by the route add command or in the service console Routing and Remote Access Service ;
dynamic entries, created by different routing protocols (RIP or OSPF).

Let's look at two examples: the routing table of a typical workstation, located on the company’s local network, and a routing table for a server that has several network interfaces.

Work station.

In this example, there is a workstation with Windows system XP, with one network adapter and the following TCP/IP protocol settings: IP address -192.168.1.10, subnet mask - 255.255.255.0, default gateway - 192.168.1.1.

Let's enter in command line systems Windows command route print, the result of the command will be the following screen ( rice. 4.6; the text for English version systems):

Rice. 4.6.

List of interfaces- a list of network adapters installed on the computer. Interface MS TCP Loopback interface is always present and is intended to refer the node to itself. Interface Realtek RTL8139 Family PCI Fast Ethernet NIC- LAN card.

Network address- the range of IP addresses that are reachable using this route.

Network mask- subnet mask to which the packet is sent using this route.

Gateway address- IP address of the node to which packets corresponding to this route are forwarded.

Interface- designation network interface of this computer, to which packets corresponding to the route are forwarded.

Metrics- conditional cost of the route. If there are several routes for the same network, then the route with the minimum cost is selected. Typically, the metric is the number of routers a packet must go through to get to the desired network.

Let's analyze some rows of the table.

The first row of the table corresponds to the default gateway value in the TCP/IP configuration of this station. The network with address "0.0.0.0" represents "all other networks that do not match other rows in this routing table."

The second line is the route for sending packets from the node to itself.

The third line (network 192.168.1.0 with mask 255.255.255.0) is the route for sending packets to local IP network(i.e. the network in which this workstation is located).

The last line is the broadcast address for all hosts on the local IP network.

Last line on rice. 4.6- list of permanent workstation routes. These are static routes that are created with the route add command. In this example there is no such static route.

Now consider a server running Windows 2003 Server, with three network adapters:

Adapter 1 - located in the company’s internal network (IP address - 192.168.1.10, subnet mask - 255.255.255.0);
Adapter 2 - located in external network Internet provider ISP-1 (IP address - 213.10.11.2, subnet mask - 255.255.255.248, closest interface in the provider's network - 213.10.11.1);
Adapter 3 - located in the external network of the Internet provider ISP-2 (IP address - 217.1.1.34, subnet mask - 255.255.255.248, closest interface in the provider's network - 217.1.1.33).

IP networks of providers are conditional, IP addresses are chosen for illustration purposes only (although a coincidence with any existing network is quite possible).

In addition, the server has the Routing and Remote Access Service installed for routing control packets between IP networks and access to the company network through a modem pool.

In this case, the route print command will produce the routing table shown in rice. 4.7.

Rice. 4.7.

The table shows three interfaces in the list network adapter different models, loopback adapter (MS TCP Loopback interface) And WAN (PPP/SLIP) Interface- interface for accessing the network via a modem pool.

Let us note the features of the route table of a server with several network interfaces.

The first row is similar to the first row in the workstation table. It also corresponds to the default gateway value in the TCP/IP configuration of this station. Note that you can set the "Default Gateway" parameter on only one interface. In this case, this parameter was set on one of the external interfaces (the same value is reflected at the end of the table in the “Default gateway” line).

Like a workstation, each interface has routes for both unicast-packets, and for broadcast (broadcast) for each subnet.

The second line contains static route configured in the console , to forward packets to the network196.15.20.16/24.

Support for routing tables.

There are two ways to maintain the current state of routing tables: manual and automatic.

The manual method is suitable for small networks. In this case, static entries for routes are manually entered into the routing tables. Entries are created either with the route add command or in the console Routing and Remote Access Services.

IN large networks manual method becomes too labor-intensive and prone to errors. Automatic construction and modification of routing tables is carried out by the so-called "dynamic routers". Dynamic routers monitor changes in the network topology and make necessary changes into routing tables and exchange this information with other routers running the same routing protocols. IN Windows Server implemented dynamic routing V Routing and Remote Access Service. This service implements the most common routing protocols - RIP protocol versions 1 and 2 and the OSPF protocol.

I think all the people here are progressive enough to shop in online stores in China, but does everyone know that you can shop on Aliexpress much cheaper using cashback services? For those who don’t know, I’ll briefly note that by ordering a product using a link provided by a cashback service, you can return up to 8% of the amount of your order. So on AliExpress’s birthday, I want to talk in detail about the best services and compare the conditions they currently offer for shopping in China’s largest online store. And for those who are not interested in details, but only the percentage of return is important - at the end there is a plate with data.

To begin with, let me remind you about the very mechanics of the process, thanks to which such an opportunity appeared in the first place.

That is, in this case, Aliexpress.com pays commissions to all these services for the fact that they are engaged in attracting buyers to them. The percentage paid depends on the turnover of the service: the more the online store is interested in the partner, the better conditions it gives him, and when Aliexpress pays a commission to the service, the service transfers a certain percentage to the buyer. This is cashback.

Over the past few years, many cashbackers have appeared, but there is no point in reviewing them all, since most of them are completely uncompetitive. I chose only those who, with the help of their turnover, are able to get the maximum percentage from the store and after that do not become greedy, but give most of it to the buyer.

So here's who I chose:
1.
2.
3.
4. Cashback Epn
5.Kopikot
6. Dronk Cashback

1. Cashback

Those who are not yet familiar with Letyshops sometimes think that this is a scam or definitely some kind of deception, because you can’t give so much money back to people. I can confidently refute all these suspicions, since both my personal experience with Letyshops and reviews online evoke only positive emotions. And the statistics on the growth of these guys over the years helps to completely dispel doubts. recent months, which is provided by Similarweb.

You can easily register with the service in a couple of clicks through social networks, after which the tips will tell you everything you need in a couple of minutes and even suggest making your first test order in a pseudo-store. At the first stage, you will most likely find out that you need to add Letyshops to the exceptions or temporarily disable the adblock. This requirement is necessary, since the adblock can block the transfer of information to Aliexpress that you came from exactly here. That is, if you do not pass on this information, Ali simply will not pay commissions to the service, and you will not receive cashback.

2. Cashback

One more is enough famous service among cashbacks you can name Cash4brands.ru and here again you will find only positive reviews about working with it. 689 stores are connected to the service, so you can shop not only in China. A quarter of a million people use this monthly, which is 6 times less than LetyShops, but still not bad. Aliexpress offers discounts from 2 to 25%, but usually range from 2 to 6%.

3. Cashback

The only foreign service on our list, and it may not be the most convenient, but it is very popular in the world, where 20 million people visit each month and more than 1,800 stores are connected to it. For purchases on AliExpress, it provides a 5% discount.

4. Cashback ePN CashBack.ru

This is a highly specialized cashback service that works only with one store - AliExpress, but offers cashback in the amount of 7% of your receipt. The interface here is as simple as possible and apparently the simplicity is captivating, because 700,000 people visit it per month, which makes it the second most visited and its growth is also gaining momentum.

5. Cashback Kopikot.ru

Another convenient service to receive cashback. A fairly well-thought-out interface that will immediately introduce you to how everything works in it. About 360 thousand. people visit it monthly, which indicates the level of trust, which is again much less than Letishops, but still worthy.

6. Cashback Dronk.Cashback

This service stands out because it not only offers you a refund, but also helps you choose the most suitable option. You can simply enter the name of the product in the search line and it will show in which stores you can buy it and exactly how much money will be returned to you for this purchase.

And if you have already chosen what and where you want to buy, then simply insert the page address into the search line and it will show you the exact amount that you can return.

The return percentage here is variable, but it is always higher than the market average, and for most products it is even the best. Let's say for most products on AliExpress the return rate is 8%.

Comparison plate

Min. withdrawal amount	AliExpress percentage	Link
500 rub.	6.5 %
0 rub.	2-25 %
5.01 $	5 %
500 rub.	5.5 %	Kopikot.ru
0.2$	7%	ePN CashBack.ru
3$	5-8%	Dronk.ru/

conclusions

I think conclusions should be drawn based on 3 parameters:
- user-friendly interface
- % of payments.
- Reliability

By interface I think we’ll compare the level of Letyshops and Dronk. Both companies seriously thought through every step, created a training mechanism, and in general it was very pleasant to be in their office.

The most favorable percentage payments via AliExpress on this moment at

Previously, when deploying a network, organizations often connected all computers and other network devices to a single IP network. All devices in the organization were assigned IP addresses with the same network part. This type of configuration is called a flat network architecture. IN small network With a small amount devices, flat architecture is not a problem. However, as the network expands, serious difficulties can arise with this configuration.

Think about how Ethernet networks devices search necessary services and devices via broadcast. As you remember, broadcast message delivered to all nodes on the network. DHCP protocol - example network service, which depends on broadcasting. Devices send broadcast requests over the network to locate a DHCP server. On a large network, this can create significant traffic that will slow down the overall network performance. Additionally, since the broadcast is performed to all devices, they need to receive and process the traffic, resulting in increased processing requirements. If the device has to handle a significant volume of broadcasts, it may even cause the device to slow down. For this reason, larger networks need to be divided into smaller subnets dedicated to smaller groups of devices and services.

The process of segmenting a network by dividing it into several smaller networks is called subnetting. These smaller networks are called subnets. Network administrators can group devices and services into subnets based on their geographic location (for example, 3rd floor of a building), organizational unit (for example, sales department), or by device type (printers, servers, WAN, etc.) or by another principle significant for the network. Subnetting can reduce overall network load and improve performance.

Note. A subnet is similar to a network, and both terms can be used interchangeably. Most networks are themselves subnets of larger blocks of addresses.

Communication between subnets

A router is necessary for the interaction of hosts from different subnets. Devices on the network use the router interface connected to their local network as the default gateway. Traffic sent to a device on a remote network will be processed by the router and sent towards the destination network. To determine whether traffic is local or remote, the router uses a subnet mask.

In the subnet space, this mechanism is implemented in a similar way. As shown in the figure, subnets form several logical networks from one block of addresses or network address. Each subnet is treated as a separate network space. Devices on the same subnet must use the address, subnet mask, and default gateway of the subnet to which they belong.

Traffic cannot travel between subnets without the use of a router. Each router interface must have an IPv4 address, network-owned or the subnet to which this interface is connected.

The importance of dividing an IP network into subnets

Plan: Address Assignment

As shown in the figure, when planning subnets, you need to consider your organization's network usage requirements and the intended subnet structure. First you need to study the network requirements. This means examining the entire network, identifying its main parts and dividing them into segments. The address distribution plan contains information about the required subnet size, the number of nodes, and the principle of assigning addresses to nodes. In addition, you need to determine the hosts that need to be allocated static IP addresses and the hosts that will be able to receive network settings via DHCP protocol.

When determining the size of the subnet, you need to estimate the number of hosts that will need IP addresses in each subnet within the partitioned private network. For example, when designing a campus network, you need to estimate the number of nodes in the administrators' local network, in the teachers' local network, and in the student local network. In a home network, you can estimate the number of nodes in the residential area local network and in the home office local network.

As mentioned earlier, the range of private IP addresses used on the local network is selected network administrator, and the choice of this range should be taken with due care. It is necessary to ensure that the number of addresses will be sufficient for the currently active nodes and for future expansion of the network. Remember the private IP address ranges:

10.0.0.0 with subnet mask 255.0.0.0

172.16.0.0 with subnet mask 255.240.0.0

192.168.0.0 with subnet mask 255.255.0.0

Based on the IP address requirements, you can determine the range or ranges of hosts to deploy. After subnetting the selected private IP address space, host addresses that meet the network requirements will be obtained.

Public addresses used to connect to the Internet are usually allocated by your telecom operator. Although the same principles of subnetting apply, this is not always the responsibility of the organization's network administrator.

Define standards for assigning IP addresses within the range of each subnet. For example:

Printers and servers will be assigned static IP addresses

Users will receive IP addresses from DHCP servers in /24 subnets

Routers are assigned the first available host addresses in the range.

Two significant factors influencing the determination of the required block of private addresses are the number of subnets required and maximum amount nodes on each subnet. Each of these address blocks will allow nodes to be distributed based on the size of the network, the number of nodes currently active or being added in the near future. IP space requirements will determine the range or ranges of hosts used.

The examples below show subnetting based on address blocks with subnet masks 255.0.0.0 and 255.255.255.0, 255.255.0.0.

Basic subnetting

Each network address contains a valid range of host addresses. All devices connected to the same network will have the IPv4 host address of that network, as well as a common subnet mask or network prefix.

The prefix and subnet mask are different ways representations of the same thing - the network part of the address.

To create IPv4 subnets, we use one or more bits from the host portion as network portion bits. To do this, we expand the subnet mask. The more bits are borrowed from the host part, the more subnets can be created. For each bit borrowed, the number of available subnets doubles. For example, if you borrow one bit, you can create two subnets. For two bits - 4 subnets, for three bits - 8 subnets, etc. However, with each borrowed bit, the number of host addresses in each subnet decreases.

Bits can only be borrowed from the node part of the address. The network part of the address is allocated by the telecom operator and cannot be changed.

Note. In the example figures, only the last octet is displayed in binary format, since only bits from the host part can be used.

As shown in Figure 1, the 192.168.1.0/24 network has 24 bits in the network part and 8 bits in the host part, which is indicated by the subnet mask 255.255.255.0 or the /24 prefix entry. Without subnetting, this network supports only one local network interface. If an additional local network is needed, the main network must be divided into subnets.

In Figure 2, the most significant bit (the leftmost bit) borrows 1 bit from the node portion, expanding the network to 25 bits. This creates two subnets: the first is defined by the digit 0 in the borrowed bit, and the second is defined by the digit 1 in the borrowed bit. The subnet mask of both networks uses a 1 in the borrowed bit to indicate that the bit is now part of the network portion of the address.

As shown in Figure 3, if we convert the binary octet to decimal format, we see that the first subnet address is 192.168.1.0 and the second subnet address is 192.168.1.128. Since the bit was borrowed, the subnet mask for each subnet will be 255.255.255.128 or /25.

Subnets used

In the example above, the 192.168.1.0/24 network was divided into two subnets:

192.168.1.128/25

Please note that in Figure 1, two local network segments are connected to the GigabitEthernet interfaces of router R1. Subnets will be used for segments connected to these interfaces. To act as a gateway for devices on a local network, each of the router's interfaces must be assigned an IP address within the range of valid addresses for the assigned subnet. It is recommended to use the first or last available address network range.

The first subnet (192.168.1.0/25) is used for the network connected to the GigabitEthernet 0/0 interface, and the second subnet (192.168.1.128/25) is used for the GigabitEthernet 0/1 interface. To assign an IP address to each of these interfaces, you must define a range of valid IP addresses for each subnet.

Network address- all bits are 0 in the node part of the address.

First node address- all bits are 0, as well as the rightmost bit 1 in the node part of the address.

Last node address- all bits are 1, as well as the rightmost bit 0 in the node part of the address.

Broadcast address- all bits are 1 in the node part of the address.

As shown in Figure 2, the address of the first node in the 192.168.1.0/25 network is 192.168.1.1, and the address of the last node is 192.168.1.126. Figure 3 shows that the address of the first node in the 192.168.1.128/25 network is 192.168.1.129, and the address of the last node is 192.168.1.254.

To assign the address of the first host in each subnet to the router interface for that subnet, use the command ip address in interface configuration mode, as shown in Figure 4. Please note that for each subnet the subnet mask is 255.255.255.128, which means that 25 bits are allocated for the network part of the address.

The host configuration for network 192.168.1.128/25 is shown in Figure 5. Note that the gateway IP address is the address (192.168.1.129) configured on the G0/1 interface of R1, and the subnet mask is 255.255.255.128.

Subnetting formulas

Calculation of subnets

To calculate the number of subnets, use the following formula:

2^n (where n = number of bits borrowed)

As shown in Figure 1 for the 192.168.1.0/25 example, the calculation is as follows:

2^1 = 2 subnets

Calculation of nodes

To calculate the number of nodes in one network, use the following formula:

2^n (where n = number of bits remaining in the node part of the address)

As shown in Figure 2 for the 192.168.1.0/25 example, the calculation is as follows:

Because hosts cannot use a network address or a broadcast address from a subnet, these two addresses cannot be assigned to hosts. This means that 126 (128-2) host addresses can be used in each subnet.

So in this example, borrowing one host bit for the network would create two subnets, each with 126 hosts to assign.

Creating 4 subnets

Let's consider network infrastructure, which requires three subnets.

If you use identical block addresses 192.168.1.0/24, to create at least three subnets you need to borrow a few bits from the host part. If you borrow one bit, only two subnets will be created. For creating more subnets must be borrowed more bits from the nodal part. Let's calculate the number of subnets created by borrowing two bits from the host part using the formula 2^n:

2^2 = 4 subnets

Borrowing two bits allows you to create 4 subnets, as shown in Figure 1.

As you remember, the subnet mask must change to reflect the borrowed bits. In this example, borrowing two bits will expand the mask by two bits in the last octet. In decimal format the mask is 255.255.255.192 because the last octet in binary is 1100 0000.

Use the knot calculation formula as shown in Figure 2.

Do not forget that if in the host part of the address all bits are equal to 0, then this is the address of the network itself, and if all bits are equal to 1, then it is a broadcast address. Thus, only 62 host addresses are actually available on each subnet.

As shown in Figure 3, the address of the first node in the first subnet is 192.168.1.1, and the address of the last node is 192.168.1.62.

In Fig. Figure 4 shows the ranges for subnets from 0 to 2. Remember that each host must have a valid IP address in the range defined for that network segment. The subnet assigned to the router interface will determine which segment the host belongs to.

Figure 5 shows an example configuration. In this configuration, the first network is assigned to the GigabitEthernet 0/0 interface, the second network is assigned to the GigabitEthernet 0/1 interface, and the third network is assigned to the serial network 0/0/0.

In addition, according to overall plan addressing, the address of the first node in the subnet is assigned to the router interface. Hosts on each subnet will use the router's interface address as the default gateway address.

For PC1 (192.168.1.2/26), the default gateway address will be 192.168.1.1 (the address of the G0/0 interface of router R1).

For PC2 (192.168.1.66/26), the default gateway address will be 192.168.1.65 (address of interface G0/1 of router R1).

Note. All devices on the same subnet will have an IPv4 host address from the range of host addresses and use the same subnet mask.

Creation of 8 subnets

If you use the same address block 192.168.1.0/24, you need to borrow a few bits from the host part of the address to create at least five subnets. Borrowing two bits will create only four subnets, as shown in the previous example. To create more subnets, more bits must be borrowed from the host part. Let's calculate the number of subnets created by borrowing three bits from the host part using the formula:

2^3 = 8 subnets

As shown in Figures 2 and 3, borrowing three bits creates 8 subnets. If three bits are borrowed, the subnet mask will be expanded by 3 bits in the last octet (/27), giving a subnet mask of 255.255.255.224. All devices on these subnets will use the subnet mask 255.255.255.224 (/27).

Let's apply the formula for calculating nodes:

2^5 = 32, but subtract 2 for all zeros in the host part (network address) and all ones in the host part (broadcast address).

Subnets are assigned to the network segments needed for the topology, as shown in Figure 4.

In addition, according to the general addressing plan, the address of the first host in the subnet is assigned to the router interface, as shown in Figure 5. The hosts in each subnet will use the router interface address as the default gateway address.

For PC1 (192.168.1.2/27), the default gateway address will be 192.168.1.1.

For PC2 (192.168.1.34/27), the default gateway address will be 192.168.1.33.

For PC3 (192.168.1.98/27), the default gateway address will be 192.168.1.97.

For PC4 (192.168.1.130/27), the default gateway address will be 192.168.1.129.

8)
9) Routing: static and dynamic using the example of RIP, OSPF and EIGRP.
10) Broadcast network addresses: NAT and PAT.
11) First hop reservation protocols: FHRP.
12) Security computer networks and virtual private networks: VPN.
13) Global networks and protocols used: PPP, HDLC, Frame Relay.
14) Introduction to IPv6, configuration and routing.
15) Network management and network monitoring.

P.S. Perhaps over time the list will be expanded.

Let's start, or continue, with the most popular, hackneyed and sick. These are IP addresses. Over the course of 4 articles, this concept was encountered several times, and most likely you either already understood what they are for, or googled them and read about them. But I must tell you this, because without a clear understanding it will be difficult to move on.

So an IP address is the address used by a node at the network level. It has a hierarchical structure. What does it mean? This means that each number in its writing has a certain meaning. I'll explain it very clearly good example. An example would be the number regular phone- +74951234567. The first digit is +7. This indicates that the number belongs to the Russian Federation zone. Next comes 495. This is the code for Moscow. And I took the last 7 digits randomly. These numbers are assigned to the regional zone. As you can see, there is a clear hierarchy here. That is, by the number you can understand which country or zone it belongs to. IP addresses adhere to a similarly strict hierarchy. They are controlled by the IANA organization (Internet Assigned Numbers Authority). If in Russian, then this is “Administration of Internet Address Space”. Please note that the word “Internet” is capitalized. Few people attach importance to this, so I’ll explain the difference. In English-language literature, the term “internet” is used to describe several networks connected to each other. And the term "Internet" to describe global network. So take note of this.

Despite the fact that the topic of the article is more theoretical than practical, I strongly recommend that you take it seriously, since the understanding of further topics, especially routing, depends on it. It’s no secret to anyone, I think, that we are accustomed to perceive numerical information in decimal format (numbers from 0-9). However, everything modern computers perceive information in binary (0 and 1). It doesn’t matter whether information is transmitted using current or light. All of it will be perceived by the device as whether there is a signal (1) or not (0). There are only 2 values. Therefore, a translation algorithm was invented from binary system to decimal and back. I'll start with something simple and tell you what IP addresses look like in decimal format. This entire article is devoted to IP addresses version 4. There will be a separate article about version 6. In previous articles, labs, and in life in general, you have seen something like this “193.233.44.12”. This is the IP address in decimal notation. It consists of 4 numbers, called octets, separated by dots. Each such number (octet) can take a value from 0 to 255. That is, one of 256 values. The length of each octet is 8 bits, and the total length of IPv4 = 32 bits. Now interest Ask. How will the computer perceive this address and how will it work with it?

You can, of course, type this into a calculator, of which there are plenty on the Internet, and it will convert it into binary format, but I believe that everyone should be able to translate it manually. This is especially true for those who plan to take the exam. You will have nothing at hand except paper and a marker, and you will have to rely only on your skills. That's why I'm showing you how to do it manually. The table is being built.

128	64	32	16	8	4	2	1
x	x	x	x	x	x	x	x

Instead of “x”, either 1 or 0 is written. The table is divided into 8 columns, each of which carries 1 bit (8 columns = 8 bits = 1 octet). They are arranged in order of seniority from left to right. That is, the first (left) bit is the most significant and has the number 128, and the last (right) is the least significant and has the number 1. Now I will explain where these numbers come from. Since the system is binary, and the length of the octet is 8 bits, each number is obtained by raising the number 2 to a power from 0 to 7. And each of the resulting digits is written in the table from greatest to least. That is, from left to right. From 2 to the 7th power to 2 to the 0th power. I will give a table of degrees of 2.

I think it’s now clear how the table is built. Let's now break down the address "193.233.44.12" and see what it looks like in binary format. Let's look at each octet separately. Let's take the number 193 and see what table combinations it comes from. 128 + 64 + 1 = 193.

For 44 it is 32 + 8 + 4.

The result is a long bit sequence 11000001.11101001.00101100.00001100. It is with this type that network devices work. The bit sequence is reversible. You can also insert each octet (8 characters each) into the table and get the decimal notation. I will imagine a completely random sequence and reduce it to decimal form. Let it be 11010101.10110100.11000001.00000011. I build a table and enter the first block into it.

I count 128 + 32 + 16 + 4 = 180.

Third block.

2 + 1 = 3

We collect the results of the calculations and get the address 213.180.193.3. Nothing heavy, pure arithmetic. If it’s hard and really unbearably difficult, then practice. It may seem scary at first, since many graduated 10 years ago and have forgotten a lot. But I assure you that once you get the hang of it, counting will be much easier. Well, to reinforce this, I’ll give you a few examples for you to calculate on your own (there will be answers under the spoiler, but open them only when you decide for yourself).

Task No. 1

1) 10.124.56.220
2) 113.72.101.11
3) 173.143.32.194
4) 200.69.139.217
5) 88.212.236.76
6) 01011101.10111011.01001000.00110000
7) 01001000.10100011.00000100.10100001
8) 00001111.11011001.11101000.11110101
9) 01000101.00010100.00111011.01010000
10) 00101011.11110011.10000010.00111101

Answers

1) 00001010.01111100.00111000.11011100
2) 01110001.01001000.01100101.00001011
3) 10101101.10001111.00100000.11000010
4) 11001000.01000101.10001011.11011001
5) 01011000.11010100.11101100.01001100
6) 93.187.72.48
7) 72.163.4.161
8) 15.217.232.245
9) 69.20.59.80
10) 43.243.130.61

Now IP addresses don't have to be such a scary thing and you can dive deeper into them.
Above we talked about the structure telephone numbers and their hierarchy. And at the dawn of the birth of the Internet, in the representation in which we are accustomed to seeing it, a question arose. The question was that IP addresses need to be grouped somehow and the issuance controlled. The solution was to divide the entire IP address space into classes. This solution was called classful addressing (from English Classful). It has long been outdated, but almost any book devotes entire chapters and sections to it. Cisco also does not forget about this in its educational materials talks about her. So I'll go over this topic and show you what it was all about from 1981 to 1995.

The space was divided into 5 classes. Each class was assigned a block of addresses.

Let's start with class A. If you look carefully at the table, you will notice that this block is given the most big block addresses, or to be precise, half of the entire address space. This class was intended for large networks. The structure of this class is as follows.

What's the point? The first octet, i.e. 8 bits, is reserved for the network address, and the last 3 octets (i.e., the remaining 24 bits) are assigned to hosts. Here, in order to show which piece belongs to the network and which to the hosts, it is used mask. The structure of the record is similar to that of an IP address. The difference between a mask and an IP address is that 0 and 1 cannot alternate. First there are 1, and then 0. Thus, where there is a unit, it means this is a section of the network. Below, after analyzing the classes, I will show you how to work with it. Now the main thing to know is that the class A mask is 255.0.0.0. The table also mentions some first bit and for class A it is equal to 0. This bit is precisely needed so that the network device understands which class it belongs to. It also specifies the starting and ending range of addresses. If we write ones in binary on all octets except the first bit in the first octet (there is always 0), then we get 127.255.255.255, which is the boundary of class A. For example, take the address 44.58.63.132. We know that in class A the first octet is given to the network address. That is, "44" is the network address, and "58.63.132" is the host address.

Let's talk about class B

This class was given a smaller block. And the addresses from this block were intended for medium-sized networks. 2 octets are allocated for the network address, and 2 - for the host address. Class B mask is 255.255.0.0. The first bits are strictly 10. And the rest change. Let's move on to an example: 172.16.105.32. The first two octets for the network address are “172.16”. And the 3rd and 4th ones for the host address are “105.32”.

Class C

This class was deprived of addresses and given the most small block. It was intended for small networks. But this class gave as many as 3 octets for the network address and only 1 octet for the hosts. His mask is 255.255.255.0. The first bits are 110. In the example it looks like this - 192.168.1.5. The network address is “192.168.1” and the host address is “5”.

Classes D and E. I combined them into one for a reason. Addresses from these blocks are reserved and cannot be assigned to networks or hosts. Class D is for multicast. An analogy can be made with television. A TV channel broadcasts its broadcast to a group of people. And those who are connected can watch TV shows. That is, only the first 3 classes can be at the disposal of administrators.

Let me remind you that the first bits of class D are 1110. An example address is 224.0.0.5.

And the first bits of class E are 1111. Therefore, if you suddenly see an address like 240.0.0.1, feel free to say that this is an E class address.

We mentioned classes. Now I will voice a question that I was recently asked. So why then masks? Our hosts understand what class they are in. But here's the point. For example, you have a small office and you need a block of IP addresses. Nobody will give you all class C addresses. They will only give you a piece of it. For example, 192.168.1.0 with a mask of 255.255.255.0. So this mask will determine your boundary. We have already said that the octet varies in value from 0 to 255. This 4 octet is completely at your disposal. Except for the first address and the last, that is, 0 and 255 in this case. The first address is the network address (in this case 192.168.1.0) and the last address is the broadcast address (192.168.1.255). Let me remind you that a broadcast address is used when it is necessary to transmit information to all nodes on the network. Therefore there is a rule. If you need to find out the network number, then turn all the bits related to the host to 0, and if it is broadcast, then all the bits are set to 1. Therefore, if 2 addresses are taken from 256 addresses, then 254 addresses remain for assignment to hosts (256 - 2) . During interviews and exams they often ask: “How many IP addresses are there on the network?” and “How many available IP addresses are there on the network to assign to hosts?” Two different questions that can be confusing. The answer to the first question will be all addresses, including the network address and broadcast address, and the answer to the second question will be all addresses except the network address and broadcast address.

Now let's delve deeper into the study of the mask.

I wrote down the class C address 192.168.1.1 with mask 255.255.255.0 in decimal and binary format. Notice what the IP address and mask look like in binary format. If the IP address alternates 0 and 1, then the mask contains 1 first and then 0. These bits fix the network address and set the size. From the table above we can conclude that in binary form the mask is represented by a sequence of 24 units in a row. This means that as many as 3 octets are allocated for the network, and octet 4 is free for addressing for hosts. Nothing unusual here. This is a standard Class C mask.

But here's the rub. For example, your office has 100 computers, and you do not plan to expand. Why create a network of 250+ addresses that you don’t need?! Subnetting comes to the rescue. This is very convenient thing. I’ll explain the principle using the example of the same class C. No matter how much you want, you cannot touch 3 octets. They are fixed. But octet 4 is free for hosts, so you can touch it. By borrowing bits from the host chunk, you split the network into the nth number of subnets and, accordingly, reduce the number of addresses for hosts in it.

Let's try to make this a reality. I change my mask. I borrow the first bit from the host part (that is, I set the 1st bit of the 4th octet to one). This results in the following mask.

This mask divides the network into 2 parts. If before splitting the network had 256 addresses (from 0 to 255), then after splitting each piece will have 128 addresses (from 0 to 127 and from 128 to 255).
Now I’ll see what changes in general with addresses.

In red I showed those bits that are fixed and cannot be changed. That is, the mask sets a boundary for it. Accordingly, the bits marked in black are designated for addressing hosts. Now I will calculate this boundary. To determine the beginning, you need to turn all free bits (marked in black) to zero, and to determine the end, turn them to ones. I'm getting started.

That is, in the fourth octet, all bits except the first are changed. It is rigidly fixed within this network.

Now let's look at the second half of the network and calculate its addresses. Our division was carried out by borrowing the first bit in the 4th octet, which means it is a divisor. The first half of the network was obtained when this bit took the value 0, which means the second network is formed when this bit takes the value 1. I turn this bit to 1 and look at the boundaries.

I'll put it in decimal form.

Accordingly, .128 and .255 cannot be assigned to hosts. This means there are 128-2=126 addresses available.
This is how you can control the size of the network using a mask. Each borrowed bit divides the network into 2 parts. If we bite off 1 bit from the host part, we will divide it into 2 parts (128 addresses each), 2 bits = 4 parts (64 addresses each), 3 bits = 8 (32 addresses each) and so on.

If you have calculated the number of bits allocated to hosts, then the number of available IP addresses can be calculated using the formula

Classless Inter-Domain Routing or CIDR. It was described in the RFC1519 standard in 1993. She abandoned class boundaries and a fixed mask. Addresses are divided only into public and reserved, which are described above. If in classful addressing the mask was cut into one for all subnets, then in classless addressing each subnet can have its own mask. Everything is fine and dandy in theory, but there is nothing better than practice. Therefore, I’ll move on to it and explain how you can divide it into subnets with different amounts hosts.

As a cheat sheet, I will provide a list of all possible masks.

Let's imagine the situation. You were given the network 192.168.1.0/24 and set the following conditions:

1) Subnet with 10 addresses for guests.
2) Subnet with 42 addresses for employees.
3) Subnet with 2 addresses to connect 2 routers.
4) Subnet with 26 addresses for the branch.

OK. This mask shows that we have 256 addresses at our disposal. According to the condition, this network must somehow be divided into 4 subnets. Let's try. 256 is very well divided by 4, giving the answer 64. This means that one large block of 256 addresses can be divided into 4 equal blocks of 64 addresses each. And everything would be fine, but it gives rise to big number empty addresses. For employees who need 42 addresses, okay, maybe in further company will still hire. But a subnet for routers that requires only 2 addresses will leave 60 empty addresses. Yes, you can say that these are private addresses, and who cares about them. Now imagine that these are public addresses that are routed on the Internet. There are already few of them, but here we will still discard them. This is not the case, especially when we can flexibly manage the address space. Therefore, we return to the example and cut the subnets as we need.

So, what subnets should be cut to accommodate all the addresses specified by the condition?!

1) For 10 hosts, the smallest subnet will be a block of 16 addresses.
2) For 42 hosts, the smallest subnet will be a block of 64 addresses.
3) For 2 hosts, the smallest subnet will be a block of 4 addresses.
4) For 26 hosts, the smallest subnet will be a block of 32 addresses.

I understand that not everyone can get into it the first time, and there’s nothing wrong with that. All people are different and perceive information differently. To complete the effect, I will show the division in the picture.

Here we have a block consisting of 256 addresses.

After dividing into 4 parts the following picture is obtained.

We found out above that in this situation, addresses are not used rationally. Now notice how the address space looks after cutting subnets of different lengths.

As you can see, in free access There are a bunch of addresses left that we can use in the future. You can count exact figure. 256 - (64 + 32 + 16 + 4) = 140 addresses.

That's how many addresses we saved. Let's move on and answer the following questions:

What will the network and broadcast addresses be?
- What addresses can be assigned to hosts?
- What will the masks look like?

The mechanism for dividing into subnets with different masks is called VLSM (Variable Length Subnet Mask) or variable length subnet mask. I'll give important advice! Start addressing with the largest subnet. Otherwise, you may end up with addresses starting to overlap. So first plan your network on paper. Draw it, depict it in the form of figures, calculate it manually or on a calculator, and only then proceed to setting it up in combat conditions.

So, the largest subnet consists of 64 addresses. Let's start with it. The first address pool will be as follows:

The subnet address is 192.168.1.0.
The broadcast address is 192.168.1.63.
Pool of addresses to assign to hosts from 192.168.1.1 to 192.168.1.62.
Now choosing a mask. Everything is simple here. Subtract from the whole network We write the required piece and the resulting number into the mask octet. That is, 256 - 64 = 192 => mask 255.255.255.192 or /26.

The subnet address is 192.168.1.64.
The broadcast address is 192.168.1.95.
The pool of addresses to assign to hosts will be from 192.168.1.65 to 192.168.1.94.
Mask: 256 - 32 = 224 => 255.255.255.224 or /27.

The 3rd subnet, which is intended for the branch, will start from p.96:

The subnet address is 192.168.1.96.
The broadcast address is 192.168.1.111.
The pool of addresses to assign to hosts will be from 192.168.1.97 to 192.168.1.110.
Mask: 256 - 16 = 240 => 255.255.255.240 or /28.

Well, for the last subnet, which will go under the interfaces connecting the routers, it will start with 112:

The subnet address is 192.168.1.112.
The broadcast address is 192.168.1.115.
The allowed addresses will be 192.168.1.113 and 192.168.1.114.
Mask: 256 - 4 = 252 => 255.255.255.252 or /30.

Note that 192.168.1.115 is the last address used. Starting from 192.168.1.116 and up to .255 are free.

In this way, using VLSM or variable length masks, we economically created 4 subnets with the right amount addresses in each. I think this is worth fixing as a problem to solve on your own.

Task No. 3

Divide the 192.168.1.0/24 network into 3 different subnets. Find and record in each subnet its addresses, broadcast address, pool of addresses allowed for issuance, and mask. I indicate the required subnet sizes:

1) Subnet for 120 addresses.
2) Subnet for 12 addresses.
3) Subnet for 5 addresses.

Answer

1) Subnet address - 192.168.1.0.
The broadcast address is 192.168.1.127.
The pool of addresses to assign to hosts will be from 192.168.1.1 to 192.168.1.126.
Mask: 256 - 128 = 128 => 255.255.255.128 or /25.

2) Subnet address - 192.168.1.128.
The broadcast address is 192.168.1.143.
The pool of addresses to assign to hosts will be from 192.168.1.129 to 192.168.1.142.
Mask: 256 - 16 = 240 => 255.255.255.240 or /28.

3) Subnet address - 192.168.1.144.
The broadcast address is 192.168.1.151.
The pool of addresses to assign to hosts will be from 192.168.1.145 to 192.168.1.150.
Mask: 256 - 8 = 248 => 255.255.255.248 or /29.

Now that you know how to divide networks into subnets, it's time to learn how to combine subnets into one common subnet. Otherwise it's called summation or summarization. Summation is most often used in routing. When you have several neighboring subnets in the router table, routing of which passes through the same interface or address. Most likely, this process is better explained when analyzing routing, but given that the topic of routing is already a large one, I will explain the summation process in this article. Moreover, summation is pure mathematics, and in this article we deal with it. Well, I'll get started.

Let's imagine that I have a company consisting of a main building and buildings. I work in the main building, and my colleagues work in the buildings. Although I have a main building, it only has 4 subnets:

192.168.0.0/24
- 192.168.1.0/24
- 192.168.2.0/24
- 192.168.3.0/24

Then colleagues from the next building came to their senses and realized that their router configuration had gone wrong and there were no backups. They don’t remember by heart which subnets are in the main building, but they remember that they are next to each other, and ask to send one summarized one. Now I have a problem how to summarize them. First, I will convert all subnets into binary form.

Look carefully at the table. As you can see, 4 subnets have the same first 22 bits. Accordingly, if I take 192.168.0.0 with a mask /22 or 255.255.252.0, I will cover my 4 subnets. But pay attention to subnet 5, which I specifically entered. This is the subnet 192.168.4.0. Its 22nd bit is different from the previous 4, which means the above selection will not cover this subnet.
OK. Now I will send the summarized subnet to my colleagues, and if they enter everything correctly, then routing to my subnets will work without problems.

Let's take the same example and change the conditions a little. We were asked to send a summary route for subnets 192.168.0.0 and 192.168.1.0. I won't be lazy and create another table.

Please note that the first 2 subnets have the same 23 bits, not 22 bits. This means that they can be summed up even more compactly. In principle, it will work either way. But as one advertisement said: “If there is no difference, why pay more?” Therefore, try to summarize without affecting neighboring subnets.

Thus, by converting the subnets into binary format and finding the same bits, you can sum them up.

In general, summation is useful when you need to combine several subnetworks located close to each other. This will save router resources. However, this is not always possible. It is impossible to sum up, for example, the subnets 192.168.1.0 and 192.168.15.0 without including adjacent subnets. Therefore, before summing it is worth thinking about its feasibility. Therefore, I repeat once again that any revolution must be started on paper. Well, to consolidate the material, I’ll leave a small task.

Task No. 4

4 subnets are given:

1) 10.3.128.0
2) 10.3.129.0
3) 10.3.130.0
4) 10.3.131.0

Sum up the subnets and find a mask that can cover them without affecting neighboring subnets.

Answer

Based on this, the answer would be 10.3.128.0/22 (255.255.252.0)

It's time to call it a day. The article was not very long. I would even say the opposite. But we’ve covered everything Cisco needs to know about IPv4. The most important thing that is required of you is to learn how to work with addresses and masks and be able to convert them from decimal to binary and vice versa. And, of course, it is correct to divide into subnets and distribute the address space. Thank you for reading. And if you also solved the problems yourself, then there is no price for you) And if you haven’t solved them yet, then have a pleasant time.

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