Sub-Network Division

1. IP Address classification

IP addresses are unique and can be divided into 5 categories depending on the nature of the user. 　　

The first digit of the Class A address is 0;

The first two bits of the Class B address are 10;

The first three bits of the class C address are 110;

The first four bits of the Class D address are 1110;

The first five bits of the E-class address are 11110.

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Class A address network number range: 1.0.0.0---126.0.0.0

Class B Address Network number range: 128.0.0.0---191.255.0.0.

Class C Address Network number range: 192.0.0.0---223.255.255.0

Class D address range: 224.0.0.0---239.255.255.255

Class E address range: 240.0.0.0---255.255.255.254

IP address If you use only the ABCDE class to divide, it will cause a lot of waste: A network with 500 hosts, unable to use the C-class address. However, if a class B address is used, only 500 of the 60,000 + host addresses are used, resulting in a significant waste of IP addresses. Therefore, the IP address also supports VLSM technology, which can be further divided into subnets based on the ABC network.

2. Subnet mask

In order to identify the network part and the host part of the IP address, to be combined with the address mask, the mask is also the same as the IP address of bits, in dotted decimal notation. The IP address network portion of the corresponding mask portion is all "1", the host part of the mask is "0".

By default, if no subnetting is made, the subnet mask for Class A networks is the subnet mask for the 255.0.0.0,b class network and the subnet mask for the 255.255.0.0,c class network is 255.255.255.0. With subnets, the use of network addresses is more effective.

With the subnet mask, the IP address is identified by the following method:

Example: 192.168.1.1 255.255.255.0 or identified as 192.168.1.1/24 (number of "1" in The Mask)

3, subnetting

      For example: A company obtains a class C IP address from telecommunications: 120.237.116.0 , the network needs to have 5 subnets, in addition to consider 2 as a reservation, the total number of subnets required is 7. Obviously, the choice subnet number bit length is 3 (23=8) can satisfy the user request (namely 120.237.116.0 /27 ).

Network number			Sub-network number	Host number
11111111	11111111	11111111	111	00000

We choose to use the subnet mask 255.255.255.224, then the subnet partition of the IP address subnet number is 3 bits, the host number is 5 bits, its structure should be: 3-bit subnet number indicates that the network allows 8 subnets, 5-bit host number indicates that each subnet can have 30 hosts.

　　Each network, the first IP address (that is, the host portion of all 0 IP) and the last IP (that is, the host portion of all 1 IP) cannot be assigned to the host, so the number of available IP addresses per subnet is the total number of IP addresses minus 2

Using the above subnetting scheme, the IP addresses available for this network are:

subnet 1:120.237.116 . 1~120.237.116 .

subnet 2:120.237.116 . 33~120.237.116 .

subnet 3:120.237.116 . 65~120.237.116 . 94

subnet 4:120.237.116 . 97~120.237.116 . 126

subnet 5:120.237.116 . 129~120.237.116 . 158

subnet 6:120.237.116 . 161~120.237.116 .

subnet 7:120.237.116 . 193~120.237.116 . 222

Subnet 8:120.237.116. 225~120.237.116. 254

Sub-Network Division