Classes of IP-addresses.

IP - a communication protocol used by the network the smallest of the two devices to the global information network.IP-address - a unique identifier of a particular node (device) allocated to a certain network.

Recording IP-addresses

address looks like a 32-bit number in the range from 0 to 4294967295. This means that throughout the Internet may contain more than 4 billion unique addresses completely facilities.If the record addresses in binary or decimal form, it causes inconvenience by its or their memorizing treatment.Therefore, to simplify the writing of these addresses, it was decided to divide the full address into four octets (8-bit number), separated by periods.For example: the address which is in hexadecimal looks like S0290612, recorded IP-addresses will look like the smallest address - four zero, and the maximum - four groups of 255. Next region (the one that is located on the left side groups of digits by any of the dividing points) occupied area addresses younger area

(the right side of the same division point) indicates the number of the interface in the network.Position of the boundary between the host and the network portion depends on the number of bits that were taken to the network number is different, the separation is only an octet boundary (points between them), and allows you to define classes of IP-addresses.

Class model addresses

Decades addresses are split into 5 classes.It expires at the moment, the division called polnoklassovoy addressing.Classes of IP-addresses are called letters of the alphabet from A to E. Classes A to E make it possible to define identifiers for 128 networks with 16 million of network interfaces each, 16,384 network devices and 64 thousand to 2 million networks with 256 interfaces.Class D IP-based networks are provided for multicast packets in which the messages are sent to multiple hosts simultaneously.Addresses that are the initial bits 1111 are reserved for future use.

Below is a table of IP-addresses.Classes are determined by the most significant bits of addresses.

Class A

IP-addresses of class A are characterized by high bit zero and eight-bit address size belonging to the network.Written as:

Based on this, the greatest number of Class A networks can be 27, but each of them will have an address space of 224 devices.Since the first bit of the address is 0, the IP-addresses of all class A will be in the high-order octet range from 0 to 127, which, besides, would also be a network number.This address is zero and 127 are reserved for business addresses, so they can not use.For this reason, the exact number of Class A networks equals 126.

Under the node addresses in a Class A network assigned 3 bytes (or 24 bits).A simple calculation shows that it is possible to place 16,777,216 binary combinations (interface address).Since the addresses consisting entirely of zeros and ones, are specialized, the number of Class A networks is reduced to 16,777,214 addresses.

Classes B and C

main feature of IP-addresses of the class b is the value of the two high-order bits equal to 10. The size of the network will be equal to 16 bits.The format of the address of the network looks like this:

For this reason, the greatest number of Class B networks can be 214 (16384) with the address space 216 each.IP-address of the Class B start in the range of 128 to 191. It is a feature, which can be determined by the network belonging to this class.Two bytes reserved for addresses of these networks, minus zero and consisting of units of addresses, can make the number of units equal to 65 534.

Any IP-address of a class C begins in the range of 192 to 223, while the number of the network takes three senioroctet.Schematically, the address has the following structure:

three bits are the first 110, the network portion of 24 bits.The largest number of networks in this class is 221 (this network 2097152).Under the node addresses in the IP-network address assigned to the class C 1 byte, it's 254 host.

Additional network classes

in classes D and E are included with network octet above 224. These addresses are reserved for special purposes, such as, for example, multicast - Datagram certain groups of nodes in the network.

range Class D is used for sending packets, and lies in the range from to class E is reserved for future use.It includes addresses from to, if you do not want problems addressed, it is advisable not to take the IP-addresses of these ranges.

reserved IP-addresses

There are addresses that can not be given to any devices, whatever the IP-addressing.Tools IP-addresses have a specific purpose.For example, if the network address consists of zeros, then it implies that the current node belongs to the network or a particular segment.If all the units - that is the address for the broadcasts packets.

In class A, there are two special dedicated network number 0 and 127. The address is equal to zero, it is used as the default route, and 127 indicates the addressing of himself (the loopback interface).For example, treatment of IP means that the node communicates only with itself without exiting datagram at the level of data protection.For a transport layer connection is different from the connection with the remote site, so that address feedback is often used to test network software.

Definition of the network and host identifiers

Knowing the IP-address of the device when the question arises of 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, it is a Class A network, from 128 to 191 - a class B network, from 192 to 223 - a network of Class C

to identify the network should be remembered that in the A-class is the starting number in theIP-address to B - the initial two numbers in C - initial three numbers.Other identifiers are the network interfaces (nodes).For example, IP-address is the address of Class B as the first number - 139 - greater than 128 and less than 191. Therefore, the network identifier is equal, node identifier - 54.23.


With routers and bridges have the opportunity to expand your network by adding segments to it, or divide it into smaller subnets by changing the network identifier.In this case, the subnet mask is taken, which shows a segment of the IP-address will be used as the identifier of the new subnet.If IDs are the same, you can conclude that the nodes belong to the same subnet, otherwise they will be in different subnets and their connections need a router.

Classes IP-addresses are designed so that the number of networks and hosts for a particular organization is determined in advance.By default, the organization can only expand a network with a number of networked devices.There is a certain network ID and a number of nodes having a restriction in accordance with the class of the network.With a large number of nodes will be low network bandwidth, because even in any broadcast performance will fall.

Subnet masks

To separate identity, it is necessary to use a subnet mask - a pattern that helps to distinguish the network ID from the host ID in the IP-addresses.Classes IP-address does not impose restrictions on the subnet mask.The mask looks the same as the address - the four groups of digits from 0 to 255. In this first go great number of them smaller.For example, - this is the correct subnet mask, - wrong.The mask determines the initial three octets of the IP-address as the subnet ID.

In the design of the enterprise network segmentation is necessary to properly organized IP-addressing.Classes of IP-addresses, separated into segments with masks, can not only increase the number of computers on the network, but also to organize its performance.Each class has a network mask address by default.

For additional subnet mask is not often used by default, and the individual.For example, IP-address can use a subnet mask of network ID, it is not necessary to use a subnet mask with ID, which is the default.This allows you to split the existing network of organizations in the class identifier subnets using different masks.

Dimensioning subnets

After setting the subnet on each software interface network protocol will conduct a survey IP-addresses using a 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 (the number of bits equal to one in a mask) - 2 = largest number of subnets;
  • 2 (the number of zeros in the subnet mask) - 2 = the greatest number of devices on the subnet.

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

When using the standard class B network mask as network can have 65,534 connected devices.If the subnet address takes full byte unit, the number of connected devices on each subnet is reduced to 254. If you want to exceed this number of devices can be problems to be solved by shortening the field mask subnet or adding another secondary address to the router interface.But in this case there will be a reduction in the number of possible networks.

When you create subnets in a Class C network, be aware that the choice will be very small at only free one octet.When sifting zero and the broadcast address is the possibility of creating four sets of optimal variants of subnets, one subnet 253 host two subnets 125 hosts four subnets on 61 host eight subnets of 29 hosts.Other variants of the partition will cause problems in routing broadcasts or simply to cause inconvenience in the calculation of addressing between hosts.

Forming subnets in a Class B network is easier, as more freedom of choice.By default, the subnet mask is, while using it get 65534 host.When creating a subnet mask under their addresses are allocated from the left untagged bits 3 and 4 octets.Can be derived by calculating the optimal network with the numbers 32, 64, 96, 128, 160 and 192.

Class A network has a very large number of addresses, to which it is possible to create a subnet.To use the subnet masks, you can use up to 32 bits.Using the above formula, we can determine the maximum number of subnets that can be up to 254. In this case, in the host addresses is 16 bits, it is possible to connect 65534 nodes.

Of course, this is only approximate calculations.When creating sectors and working with subnets must take into account more factors, which are dependent on the provider and the level of the enterprise.