Classes of IP-addresses. Class A, B, C IP addresses

IP is a communication protocol that is used from the smallest network of two devices to the global information network. An IP address is a unique identifier of a specific node (device) that is allocated in a specific network.

IP Address Recording

The address looks like a 32-bit number in the range from 0 to 4294967295. This indicates that the entire Internet network can contain more than 4 billion completely unique addresses of objects. If you write addresses in binary or decimal form, then this causes your inconvenience in storing or processing them. Therefore, to simplify the writing of such addresses, it was decided to divide the entire address into four octets (8-bit numbers) separated by a period. For example: the address in hexadecimal system looks like C0290612, in the IP address record it will look like 192.41.6.18. In this case, the smallest address is four zeros, and the maximum is four groups of 255. The higher region (the one that is located on the left side of the groups of digits from any of the dividing points) is occupied by the address area, the lower region (on the right side of the same separating point ) Shows the interface number in this network. The position of the boundary between the host and the network parts depends on the number of bits that are allocated to the network number, it can be different, the separation only goes along the border of the octet (the points between them) and allows you to define IP address classes.

Class address model

For several decades, addresses have a division into 5 classes. This outdated division is called full-class addressing. Classes of IP addresses are called letters of the Latin alphabet from A to E. Classes A through E provide the ability to specify identifiers for 128 networks with 16 million network interfaces in each, 16384 networks with 64 thousand devices and 2 million networks with 256 interfaces. Classes of IP-networks D are provided for multicasting, in which message packets are sent to several hosts simultaneously. The addresses that have the start bits 1111 are reserved for future use.

Below is the table of IP addresses. Classes are determined by the highest bits of addresses.

Class A

Class A IP addresses are characterized by a zero highest address bit and an eight-bit network size. Recorded in the form:

Based on this, the largest number of Class A networks can be 2 ⁷ , but each of them will have an address space of 2 ²⁴ devices. Since the first bit of the address is 0, all IP addresses of class A will be in the range of the highest octet from 0 to 127, which, moreover, will be the network number. In this case, the zero address and 127 are reserved for service addresses, so they can not be used. For this reason, the exact number of Class A networks is 126.

The address of the nodes in the Class A network is 3 bytes (or 24 bits). A simple calculation shows that you can place 16 777 216 binary combinations (interface addresses). Since addresses consisting entirely of zeros and ones are specialized, the number of Class A networks decreases to 16,777,214 addresses.

Classes B and C

The main distinguishing feature of the IP address of class b is the value of the two highest bits, equal to 10. In this case, the size of the network part will be equal to 16 bits. The address format of this network looks like this:

For this reason, the largest number of networks of class B can be 2 ¹⁴ (16384) with address space 2 ¹⁶ each. Class B IP addresses start in the range from 128 to 191. This is the distinctive feature by which you can determine whether the network belongs to this class. Two bytes allocated to the addresses of these networks, minus zero and consisting of units of addresses, can make up the number of nodes equal to 65,534.

Any Class C IP address starts in the range of 192 to 223, with the network number occupying three senior octets. Schematically, the address has the following structure:

The three most significant bits are first 110, the network part is 24 bits. The largest number of networks in this class is 2 ²¹ (this is 2,097,152 networks). The address of the nodes in the IP address of Class C networks is 1 byte, which is only 254 hosts.

Additional Network Classes

Classes D and E include networks with a higher octet above 224. These addresses are reserved for specialized purposes, such as, for example, multicasting-sending datagrams to specific groups of nodes on the network.

The range of class D is used for sending packets and lies in the range from 224.0.0.0 to 239.255.255.255. The last class, E, is reserved for future use. It includes addresses from 240.0.0.0 to 255.255.255.255. Therefore, if you do not want problems with addressing, it is advisable not to take IP addresses from these ranges.

Reserved IP Addresses

There are addresses that can not be given to any devices, no matter what the IP addressing is. Service IP addresses have a specific purpose. For example, if the network address consists of zeros, then this implies that the node belongs to the current network or to a specific segment. If all units are, then this is the address for broadcast packets.

In class A there are two dedicated special networks with numbers 0 and 127. The address equal to zero is used as the default route, and 127 shows the addressing to itself (the feedback interface). For example, accessing IP 127.0.0.1 means that the node communicates only with itself without the output of datagrams to the level of the data transfer medium. For the transport layer, such a connection does not differ from the connection with the remote node, so this feedback address is often used to test network software.

Identify the network and node identifiers

Knowing the IP address of the device in case there is a question about how to determine the class of IP-addresses, it is enough just to look at the first octet of the address. If it is from 1 to 126, then it is a network of class A, from 128 to 191 it is a network of class B, from 192 to 223 - a network of class C.

To identify the network, remember that in A class this is the initial number in the IP address, in B - the initial two numbers, in C - the initial three numbers. The rest are identifiers of network interfaces (nodes). For example, the IP address 139.17.54.23 is the address of class B, since the first number is 139 - more than 128 and less than 191. Therefore, the network ID will be 139.17.0.0, the node ID is 54.23.

Subnets

With the help of routers and bridges, you can expand the network by adding segments to it, or divide it into smaller subnets by changing the network ID. In this case, the subnet mask is taken, which shows which segment of the IP address will be used as the new ID of the subnet. If the identifiers match, you can conclude that the nodes belong to the same subnet, otherwise they will be on different subnets and they will need a router to connect them.

IP address classes are designed so that the number of networks and nodes for a particular organization is determined in advance. By default, in an organization, you can deploy only one network with a number of devices connected to the network. There is a specific network identifier and a number of nodes that have a restriction in accordance with the network class. With a large number of nodes, the network will have a low bandwidth, because even with any broadcast broadcast, performance will fall.

Subnet Masks

In order to separate the identifier, it is necessary to use a subnet mask - a template that helps to distinguish the network identifiers from the node identifiers in the IP addresses. IP address classes do not impose restrictions on the subnet mask. The mask looks like the address itself - four groups of digits from 0 to 255. At the same time, large numbers first follow, followed by smaller ones. For example, 255.255.248.0 is the correct subnet mask, 255.248.255.0 is the wrong one. The mask 255.255.255.0 specifies the initial three octets of the IP address as the subnet ID.

When designing the segmentation of an enterprise network, it is necessary that IP addressing is properly organized. Classes of IP addresses, divided into segments using masks, allow not only to increase the number of computers on the network, but also to organize its high performance. Each address class has a default network mask .

For additional subnets, not masks are often used, but individual ones. For example, the IP address 170.15.1.120 can use a subnet mask of 255.255.255.0 with the network ID of 170.15.1.0, and it is not necessary to use the subnet mask of 255.255.0.0 with the identifier 170.15.0.0, which is used by default. This allows you to split an existing Class B organization network with 170.15.0.0 on a subnet using various masks.

Calculation of subnetwork parameters

After configuring the subnet on each interface, the network protocol software will poll the IP addresses using the subnet mask to determine the subnet address. There are two simple formulas for calculating the maximum number of subnets and hosts on the network:

• 2 ^{(number of bits equal to one in the mask)} - 2 = the largest number of subnets;
• 2 ^{(number of zeros in the subnet mask)} - 2 = the largest number of devices on the subnet.

For example, take the address equal to 182.16.52.10 with the mask 255.255.224.0. The mask in binary form looks like this: 11111111.11111111.11100000.00000000. Judging by the first octet, this network belongs to class B, so consider the third and fourth octets. Three units and thirteen zeros are substituted into the formulas and we get 23-2 = 6 subnets and 213 - 2 = 8190 hosts.

When using the standard Class B network mask in the form 255.255.255.0, the network can have 65534 connected devices. If the subnet address occupies the full byte of the node, then the number of connected devices on each subnet is reduced to 254. If you need to exceed this number of devices, there may be problems solved by shortening the subnet address mask field or adding one more secondary address in the interface of the router. But in this case, there will be a decrease in the number of possible networks.

When creating subnets in a class C network, remember that the choice will be very small with only one octet free. When screening zero and broadcast addresses, it is possible to create four optimal subnet sets: one subnet for 253 hosts, two subnets for 125 hosts, four subnets for 61 hosts, eight subnets for 29 hosts. The remaining partitioning options will cause problems in routing and broadcasts, or simply cause inconvenience in addressing calculations between hosts.

Forming subnets in networks of class B is already easier, since there is more freedom of choice. By default, the subnet mask is 255.255.0.0, when using it, we get 65534 hosts. When creating subnet masks, the left unmarked bits of 3 and 4 octets are allocated to their addresses. By calculations it is possible to derive optimal networks with numbers 32, 64, 96, 128, 160 and 192.

Class A networks have a very large number of addresses, for which it is possible to create subnets. To use subnet masks, you can use up to 32 bits. Using the above formula, we can determine that the maximum number of subnets can be up to 254. At the same time, 16 bits are left on the host addresses, that is, 65534 nodes can be connected.

Of course, these are only approximate calculations. When creating sectors and working with subnets, it is necessary to take into account more factors that depend on the provider and enterprise level.