Code farmers should also understand the IPv4 subnet division of point networks

The original author ： A promise CTO
CTO Daily reference wechat official account
Link to the original text ： https://mp.weixin.qq.com/s/qViFY0VTaRwUMcMWC9adhQ

stay IP In the world of , Mask is a very important concept , Make an inappropriate metaphor ：IP The relationship between address and mask is like that between person and shadow , Unmasked IP It doesn't work .

current IPv4 The address is a 32 bit The integer of , In a computer, a long integer variable can be used to express . Value range ：0~4294967295(2^32-1). For the human brain , Whether it is expressed in decimal or binary 32 bit Is not a good method . therefore ,IPv4 Addresses are usually expressed in dotted decimal . Here is the same IP Address ：122.70.156.25 Different representations of ：
dotted decimal ：122.70.156.25
Decimal system ： 2051447833
Binary system ： 01111010.01000110.10011100.00011001
Obviously fractional decimal is easier to remember .

IP The address is divided into two parts , Network address （ Also known as “ Prefix ”） And host address . It's like our postal address , for example ： Chang an avenue 10 Number , In this address , Chang'an Street is equivalent to IP The network address in the address ,10 No. is equivalent to IP Host address in address , The two add up to an address that clearly describes a place .
Mask and IP Equal length , It's also 32 bit The integer of , It is mainly used to distinguish IP Network address and host address in the address , From left to right , Part of the network address is full 1, Part of the host address is full 0, And then IP Address and mask are performed bit by bit “ And ” operation , namely ：1 & 1 = 1,1 & 0 = 0.
From this we can get ：
Formula 1 ： network address = IP Address & Mask
Formula 2 ： The host address = IP Address – network address

Masks are usually described in terms of mask length , such as , The mask length is 24 Then it means mask ：255.255.255.0, The principle is that every 255 Are equal to binary 8 A continuous 1, namely ：11111111.3 individual 255 together , Equivalent to 24 A continuous 1. Similarly, the mask length is 25 Then it means mask ：255.255.255.128, front 3 individual 255 amount to 24 individual 1, Next to it 128 It's binary 10000000, Just like the one in front 24 individual 1 Together make up 25 individual 1.
From this we can get ：
Formula 3 ： The network contains IP Number of addresses = 2 ^ (32- Mask length )

meanwhile IP Provisions of the agreement , The last of each network segment IP The address is the broadcast address of the network segment , That is, the host address part is all 1 The address of . Network address and advertising address cannot be assigned to the host for use .
From this we can get ：
Formula four ： The network contains available （ Distribute ）IP Number of addresses = 2 ^ (32- Mask length ) – 2

Apply the above formula , We get IP Address ：192.168.100.100/255.255.255.0 The network address of is 192.168.100.0, The host address is 100, The network segment has 2 ^ ( 32 – 24 ) = 2 ^ 8 = 256 individual IP Address , Remove the network address and broadcast address , There are also available IP Address 256-2 = 254 individual .

Above is IP And mask , The following is how to control and divide subnets through masks , This is also IP The essence of planning …
As I said before ,IP Address includes network address and host address , In fact, through the control of mask , It can realize the hierarchical network address , That is, the concept of subnet .

Please look at this IP Address ： Beijing . Xicheng district . Chang an avenue .10 Number /255.255.255.0, Through the mask, we know that IP My network address is ：“ Beijing . Xicheng district . Chang an avenue .0”. Now let's set the mask to ：255.255.0.0, Then the network address becomes ：“ Beijing . Xicheng district .0.0”, Obviously the former is part of the latter , That is to say, the former is a subnet of the latter . If you will , You can adjust the mask to ：255.0.0.0, So the network address becomes ：“ Beijing .0.0.0” It can be used to express the whole of Beijing .

So we get this hierarchical IP The Internet ：
Beijing .0.0.0 / 8
Beijing . Xicheng district .0.0 / 16
Beijing . Xicheng district . Chang an avenue .0 /24
Beijing . Xicheng district . Fuyou Street .0 /24
Beijing . Dongcheng District .0.0 /16
…….
Empathy , Replace the above ground with numbers to get ：
192.0.0.0 / 8
192.168.0.0 / 16
192.168.100.0 /24
192.168.200.0 /24
192.169.0.0 /16
…….
It can be seen that , The division of subnets , Nothing more than adjusting the size of the network address by controlling the length of the mask , The following is an example of subnet partition .

A provincial company of a unit applies for a network segment from the headquarters 10.100.0.0/16, Provincial companies have branches in every prefecture and city , One 13 Branches . Now it is required that each prefecture and city should distribute the , The provincial company shall properly retain some IP.

specific working means ：

13 Branches + Provincial company , altogether 14 Net segment ,8<14<16, Therefore, it is necessary to divide into 16 Share （2^4）;

The length of the parent network segment mask is 16, Then this network segment contains 2^（32-16）= 65536 An address , All are divided into 16 Subnet , Then each subnet contains 65536 / 16 = 4096 individual IP The earth ;

4096 = 2^12, So the subnet mask is 32-12=20; meanwhile 4096 Expressed in dotted decimal as ：0.0.16.0

in summary , Get the following subnet list ：

10.100.0.0/20 10.100.16.0/20 10.100.32.0/20 10.100.48.0/20

10.100.64.0/20 10.100.80.0/20 10.100.96.0/20 10.100.112.0/20

10.100.128.0/20 10.100.144.0/20 10.100.160.0/20 10.100.176.0/20

10.100.192.0/20 10.100.208.0/20 10.100.224.0/20 10.100.240.0/20

We can see , The latter subnet is added to the former subnet “0.0.16.0”. Here we are , We can assign the first three network segments to provincial companies , be left over 13 Network segments are allocated to branches . The branch office gets IP after , You can also continue to subdivide . for example , A branch office got the address ：10.100.160.0/20, It can be further divided into ：10.100.160.0/22,10.100.164.0/22,10.100.168.0/22,10.100.168.0/22 Four network segments .

Reference resources

IPV4 And subnet partition _ Xiao Lu's blog -CSDN Blog _ipv4 Subnet partition