Subnet mask

A subnet mask is used to divide the network classes into even finer sections. Like the IP address, the subnet mask is represented by 4 decimal numbers separated by decimal points.

Each decimal number stands for 1 byte (8 bits) of the subnet mask and can also be represented using binary code:

Example of a subnet mask: 255.255.255.128

Byte

Decimal

Binary

1

255

11111111

2

255

11111111

3

255

11111111

4

128

10000000

The binary representation of the IP address and the subnet mask shows that in the subnet mask, all bits of the network address are set to 1 and only the bits of the node addresses have the value 0:

IP address: 192.168.10.129

Subnet mask: 255.255.255.128

 Meaning

Bytes 1 – 4

Bytes 1 – 4

Network address

11000000

11111111

10101000

11111111

00001010

11111111

Node address

10000001

10000000

The class C network with the network address 192.168.10 is further subdivided into the following 2 networks by the subnet mask 255.255.255.128:

Network address

Node addresses

192.168.10.0

192.168.10.1 – 192.168.10.126

192.168.10.128

192.168.10.129 – 192.168.10.254

The network nodes use a logical AND operation for the IP address and the subnet mask to determine whether there is a communication partner in the same network or in a different network. If the communication partner is in a different network, the standard gateway is addressed for passing on the data.