Configure TCP/IP - MCSA Windows Server 2012 R2: Installation and Configuration Study Guide - page 401

fIgure 8.6 The subnet mask revealed

Subnet mask code

1s = Positions representing network or subnet addresses

0s = Positions representing the host address

Subnet mask for Widget, Inc.

1111 1111. 1111 1111. 1111 1111.00000000

Network address
positions

Subnet
positions

Host
positions

The subnet mask can also be expressed using the decimal equivalents of the binary
patterns. The binary pattern of 1111 1111 is the same as decimal 255. Consequently, the
subnet mask in the example can be denoted in two ways, as shown in Figure 8.7.

fIgure 8.7 Different ways to represent the same mask

Subnet mask in binary: 111 111111111 111111111111 00000000 . . .

Subnet mask in decimal: 255 . 255 . 255 . 0

(The spaces in the abo ve example are only for illustrative purposes.
The subnet mask in decimal would actually appear as 255.255.255.0.)

Not all networks need to have subnets, and therefore they don't need to use custom
subnet masks. In this case, they are said to have a default subnet mask. This is basically the
same as saying that they don't have any subnets except for the one main subnet on which
the network is running. Table 8.4 shows the default subnet masks for the different classes
of networks.

Table 8.4 Default subnet masks

Class Format Default Subnet Mask

A Network.Node.Node.Node 255.0.0.0

B Network.Network.Node.Node 255.255.0.0

C Network.Network.Network.Node 255.255.255.0