Relationship between subnet number, host number and subnet mask

A few days ago, a netizen asked me some questions , How many subnets can be divided at most , How many hosts can it hold at most , How to calculate the subnet mask and so on . I told him the result without thinking , But he asked me how the result came , Detailed calculation steps , This really embarrassed me . I've been in this business for 11 Years. , Technology and pure theory have long been honed into experience in work , Some things even become instinctive and subconscious , Like a part of your body , Naturally, you can blurt out the results with ease , But how to explain it to him , I thought about it seriously for a long time . So I want to sort these things out , I hope interested friends can use it .

 

Be careful ： This blog post is a personal experience sharing , Not knowledge popularization and skill introduction , If non professionals or friends without the theoretical basis of network technology cannot understand , Please forgive me .

 

Take the actual example directly , It's easy to understand .

1、 Use the number of subnets to calculate the subnet mask

hold B Class address 172.16.0.0 Divide into 30 Subnetworks , What is its subnet mask ？

① Number of subnetworks 30 Convert to binary representation 11110

② Let's count the binary numbers 5 position

③ Be careful ： When there is only one binary number 1 When , The figures counted need to be subtracted 1（ for example :10000 Count it as 4 position ）

④ take B The subnet mask of the class address 255.255.0.0 Host address section Of front 5 position become 1

⑤ This gives you the subnet mask you want （11111111.11111111.11111000.00000000）255.255.248.0.

 

2、 Use the number of hosts to calculate the subnet mask

hold B Class address 172.16.0.0 Divided into subnetworks , Each subnet can hold 500 Console host , What is its subnet mask ？

① hold 500 Convert to binary representation 111110100

② Let's count the binary numbers 9 position

③ Mask the subnet 255.255.255.255 From the back forward Of 9 position become 0

④ This gives you the subnet mask you want （11111111.11111111.11111110.00000000）255.255.254.0.

 

3、 Use the subnet mask to calculate the maximum number of effective subnets

A class IP Address , The subnet mask is 255.224.0.0, What is the maximum number of effective subnets it can partition ？

① Convert subnet mask to binary representation 11111111.11100000.00000000.00000000

② Count its Network bit share 11 position

③A The basic number of class address network bits is 8, The difference between the two figures is 3

④ The maximum number of effective subnets is 2 Of 3 Power , That is, it can be divided into at most 8 Subnetworks .

 

4、 Use the subnet mask to calculate the maximum number of hosts available

A class IP Address , The subnet mask is 255.252.0.0, Divide it into subnetworks , How many hosts are available in each subnet ？

① Convert subnet mask to binary representation 11111111.11111100.00000000.00000000

② Count its Host bit share 18 position

③ The maximum number of hosts available is 2 Of 18 The power minus 2（ Remove all that 0 The Internet address and all are 1 Broadcast address ）, That is, each subnet has at most 262142 Hosts available .

 

5、 Use the subnet mask to determine the start and end address of the subnet

B class IP Address 172.16.0.0, The subnet mask is 255.255.192.0, What is the starting and ending address of the subnet that it can divide ？

① Use subnet mask to calculate , At most, it can be divided into 4 Subnetworks

② Use subnet mask to calculate , Each subnet can hold 16384 Console host （ Including network address and broadcast address ）

③ use 16384 Divide 256（ The total number of hosts in the network segment including network address and broadcast address ）, The result is 64

④ The specific starting and ending methods of network division are as follows ：

172.16.0.0～172.16.63.255

172.16.64.0～172.16.127.255

172.16.128.0～172.16.191.255

172.16.192.0～172.16.255.255

 

6、 The following is the relationship table of personal experience summary ,

A class IP Address segment ：1.0.0.0～126.255.255.255  Private address segment ：10.0.0.0～10.255.255.255

 

B class IP Address segment ：128.0.0.0～191.255.255.255  Private address segment ：172.16.0.0～172.31.255.255

 

C class IP Address segment ：192.0.0.0～223.255.255.255  Private address segment ：192.168.0.0～192.168.255.255

Calculate the number of subnets and the number of host addresses that can be used in each subnet through the subnet mask ：

　　 If you want to ***.***.***.0/m Divided into ***.***.***.0/n（m<n<32）, The number of subnets obtained is 2(n-m), Get rid of all 0 And all 1, The number of available subnets is 2(n-m)-2, The available host addresses in each subnet are 2(32-n)-2.

　　 for example ： Put the Internet 117.15.32.0/23 Divided into 117.15.32.0/27, The subnet you get is 16  individual , The number of available subnets is 14 individual , The available host addresses in each subnet are  30 individual .