Creating the Network Mask

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 and which represent the host number. Recall that the complete IP 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. You then need to carry out the procedures in "How to Add a Subnet to a Network". 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 "Parts of the IP Address", 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 either to the network number or the host number, depending on the network class.

For example, in a class B IP address, the 2 left-hand bytes are assigned to the network number, and the 2 right-hand bytes are assigned to the host number. In the class B IP address 129.144.41.10, you can assign the 2 right-hand bytes to hosts.

If you are going to implement subnetting, you need to use some of the bits in the bytes 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.

The bits in the host address bytes that will be applied to subnet addresses and those applied to host addresses is 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 IP address using the bitwise logical AND operator. This operation selects out the network number and subnet number positions of the address.

It is easiest to explain netmasks 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 255.255.255.0 is applied to the IP address 129.144.41.101, the result is the IP address of 129.144.41.0.

129.144.41.101 & 255.255.255.0 = 129.144.41.0

In binary form, the operation is:

10000001.10010000.00101001.01100101 (IP 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 you have a network with the number 129.144.41, that is what the system looks for and finds. Since you can assign up to 254 values to the third byte of the IP address space, subnetting lets you create address space for 254 networks, where previously there was room for only one.

If you want to provide address space for only two additional networks, you could use a subnet mask of:

255.255.192.0

This netmask provides a result of:

11111111.11111111.1100000.00000000

This still leaves 14 bits available for host addresses. Since all 0s and 1s are reserved, at least two bits must be reserved for the host number.