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For subnetting to work, the router must be told which portion of the host ID should be used for the subnet network ID. This little sleight of hand is accomplished by using another 32-bit number, known as a subnet mask. Those IP address bits that represent the network ID are represented by a 1 in the mask, and those bits that represent the host ID appear as a 0 in the mask. As a result, a subnet mask always has a consecutive string of ones on the left, followed by a string of zeros.

For example, the subnet mask for the subnet shown in Figure 3-3, where the network ID consists of the 16-bit network ID plus an additional 4-bit subnet ID, would look like this:

11111111 11111111 11110000 00000000

In other words, the first 20 bits are ones, and the remaining 12 bits are zeros. Thus, the complete network ID is 20 bits in length, and the actual host ID portion of the subnetted address is 12 bits in length.

To determine the network ID of an IP address, the router must have both the IP address and the subnet mask. The router then performs a bitwise operation called a logical AND on the IP address in order to extract the network ID. To perform a logical AND, each bit in the IP address is compared with the corresponding bit in the subnet mask. If both bits are 1, the resulting bit in the network ID is set to 1. If either of the bits are 0, the resulting bit is set to 0.

For example, here’s how the network address is extracted from an IP address using the 20-bit subnet mask from the previous example:

IP address: 10010000 00011100 00010000 00010001 (144.28.16.17)Subnet mask: 11111111 11111111 11110000 00000000Network ID: 10010000 00011100 00010000 00000000 (144.28.16.0)

Thus, the network ID for this subnet is 144.28.16.0.

The subnet mask itself is usually represented in dotted-decimal notation. As a result, the 20-bit subnet mask used in the previous example would be represented as 255.255.240.0:

Subnet mask: 11111111 11111111 11110000 00000000 255 . 255 . 240 . 0

Don’t confuse a subnet mask with an IP address. A subnet mask doesn’t represent any device or network on the Internet. It’s just a way of indicating which portion of an IP address should be used to determine the network ID.

Note that a subnet mask cannot be an arbitrary collection of octets. Instead, a subnet mask always has a certain number of binary 1s on its left side, and the remaining bits of the mask are always 0. This limits the dotted-decimal representation of a subnet mask to certain values.

You can spot a subnet mask right away because the first octet is always 255, and 255 is not a valid first octet for any class of IP address.