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 |
 | Assignment of subnets, host numbers and IP addresses Neither subnet 0 nor the subnet consisting only of ones may be used. Host number 0, and the host number consisting only of ones, must not be used. Under BootP or DHCP the subnet mask is transmitted also by the server. |