Subnet mask
The subnet mask is subject to the control of the network administrator, and specifies the structure of the subnet.
Small networks without a router do not require a subnet mask. The same is true if you do not use registered IP numbers. A subnet mask can be used to subdivide the network with the aid of the mask instead of using a large number of network numbers.
The subnet mask is a 32-bit number:
- Ones in the mask indicate the subnet part of an address space.
- Zeros indicate that part of the address space which is available for the host IDs.
Description | Binary representation | Decimal representation |
|---|---|---|
IP address | 10101100.00010000.00010001.11001000 | 172.16.17.200 |
Subnet mask | 11111111.11111111.00010100.00000000 | 255.255.20.0 |
Network ID | 10101100.00010000.00010000.00000000 | 172.16.16.0 |
Host ID | 00000000.00000000.00000001.11001000 | 0.0.1.200 |
Standard subnet mask
Address class | Standard subnet mask (decimal) | Standard subnet mask (hex) |
|---|---|---|
A | 255.0.0.0 | FF.00.00.00 |
B | 255.255.0.0 | FF.FF.00.00 |
C | 255.255.255.0 | FF.FF.FF.00 |
 | Subnets and host number Neither subnet 0 nor the subnet consisting only of ones may be used. Neither host number 0 nor the host number consisting only of ones may be used! |