When setting up the network, we all need to understand the subnet mask for network settings.
Give an example to illustrate the algorithm.
Example: Given a class c address: 192.168.5.0, it is required to divide 20 subnets, each subnet has 5 hosts.
Solution: Because 4 <5 < 8, use 256-8=248 -> that is the desired subnet mask, and the corresponding subnet number will come out. This is for Class C addresses. The teacher also only talked about the practices for C-type addresses. The following is the practice I introduced myself for Class B addresses. For Class B addresses, if the number of hosts is less than or equal to 254, it is the same as the Class C address algorithm. For those with a number of hosts greater than 254, if you need 700 hosts and 50 subnets (which are quite large), 512 < 700 < 1024
256-(1024/256)=256-4=252 ―> That is the subnet mask you are looking for, and the corresponding subnet number will be released. The 4 (the power of 2) in 256-4 above refers to the number of bits that exceed 8 digits when the host number is expressed in binary, that is, exceeding 2 digits, and the mask is the remaining first 6 digits, that is, the number of subnets is 2 (6)-2=62.
Append :Host/Subnet Quantities Table
--------------------------------------------------------------Class A
Effective Effective
# bits Mask Subnets Hosts ------- ---------------
2 255.192.0.0 2 4194302
3 255.224.0.0 6 2097150
4 255.240.0.0 14 1048574
5 255.248.0.0 30 524286
6 255.252.0.0 62 262142
7 255.254.0.0 126 131070
8 255.255.0.0 254 65536
9 255.255.128.0 510 32766
10 255.255.192.0 1022 16382
11 255.255.224.0 2046 8190
12 255.255.240.0 4094 4094
13 255.255.248.0 8190 2046
14 255.255.252.0 16382 1022
15 255.255.254.0 32766 510
16 255.255.255.0 65536 254
17 255.255.255.128 131070 126
18 255.255.255.192 262142 62
19 255.255.255.224 524286 30
20 255.255.255.240 1048574 14
21 255.255.255.248 2097150 6
22 255.255.255.252 4194302 2
Class B Effective Effective
# bits Mask Subnets Hosts
------- --------------- --------- ---------
2 255.255.192.0 2 16382
3 255.255.224.0 6 8190
4 255.255.240.0 14 4094
5 255.255.248.0 30 2046
6 255.255.252.0 62 1022
7 255.255.254.0 126 510
8 255.255.255.0 254 254
9 255.255.255.128 510 126
10 255.255.255.192 1022 62
11 255.255.255.224 2046 30
12 255.255.255.240 4094 14
13 255.255.255.248 8190 6
14 255.255.255.252 16382 2
Class C Effective Effective
# bits Mask Subnets Hosts
------- --------------- --------- ---------
2 255.255.255.192 2 62
3 255.255.255.224 6 30
4 255.255.255.240 14 14
5 255.255.255.248 30 6
6 255.255.255.252 62 2
*Subnet all zeroes and all ones excluded.
*Host all zeroes and all ones excluded.