System Administration Guide: IP Services

Creating the Network Mask for IPv4 Addresses

As part of the subnetting process, you need to select a network-wide netmask. The netmask determines how many and which bits in the host address space represent the subnet number and how many bits and which represent the host number. Recall that the complete IPv4 address consists of 32 bits. Depending on the address class, as many as 24 bits and as few as 8 bits can be available for representing the host address space. The netmask is specified in the netmasks database.

If you plan to use subnets, you must determine your netmask before you configure TCP/IP. If you plan to install the operating system as part of network configuration, the Solaris installation program requests the netmask for your network.

As described in Administering Network Numbers, 32-bit IP addresses consist of a network part and a host part. The 32 bits are divided into 4 bytes. Each byte is assigned to either the network number or the host number, depending on the network class.

For example, in a class B IPv4 address, the 2 bytes on the left are assigned to the network number, and the 2 bytes on the right are assigned to the host number. In the class B IPv4 address, you can assign the 2 bytes on the right to hosts.

If you are to implement subnetting, you need to use some of the bits in the bytes that are assigned to the host number to apply to subnet addresses. For example, a 16-bit host address space provides addressing for 65,534 hosts. If you apply the third byte to subnet addresses and the fourth to host addresses, you can address up to 254 networks, with up to 254 hosts on each network.

The bits in the host address bytes that are applied to subnet addresses and those applied to host addresses are determined by a subnet mask. Subnet masks are used to select bits from either byte for use as subnet addresses. Although netmask bits must be contiguous, they need not align on byte boundaries.

The netmask can be applied to an IPv4 address by using the bitwise logical AND operator. This operation selects out the network number and subnet number positions of the address.

Netmasks can be explained in terms of their binary representation. You can use a calculator for binary-to-decimal conversion. The following examples show both the decimal and binary forms of the netmask.

If a netmask is applied to the IPv4 address, the result is the IPv4 address of & =

In binary form, the operation is as follows:

10000001.10010000.00101001.01100101 (IPv4 address)

ANDed with

11111111.11111111.11111111.00000000 (netmask)

Now the system looks for a network number of 129.144.41 instead of a network number of 129.144. If your network has the number 129.144.41, that number is what the system checks for and finds. Because you can assign up to 254 values to the third byte of the IPv4 address space, subnetting lets you create address space for 254 networks, where previously space was available for only one.

If you are providing address space for only two additional networks, you can use the following subnet mask:

This netmask provides the following result:


This result still leaves 14 bits available for host addresses. Because all 0s and 1s are reserved, a minimum of 2 bits must be reserved for the host number.