Using the Subnet Mask

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Using the Subnet Mask

This section will look at exactly how a subnet mask is used to determine which part of the IP address is the network ID and which part is the host ID. The IP layer performs binary calculations on the IP address and the subnet mask to determine the network ID portion of the IP address.

The computation TCP/IP performs is a logical bitwise “AND” of the IP address and the subnet mask. The calculation sounds complicated, but all it really means is that the address in its true 32-bit binary format is logically “ANDed” with the subnet mask (also a 32-bit binary number). This extracts the network ID.

Performing a bitwise AND on two bits results in 1 (or TRUE), if the two values are both 1. All other cases return a 0. In the examples (tables 4.1, 4.2 and 4.3) the numbers 255 or 0 are used for the subnet mask. In binary, 255 means all the bits in that octet are 1; 0 means they are 0.

1 AND 1 results in 1

1 AND 0 results in 0

0 AND 1 results in 0

0 AND 0 results in 0

In the example in Table 4.1, the IP address 200.20.16.5 is “ANDed” with the subnet mask 255.255.255.0. Because 1 and “n” always returns “n” and because the first three octets are all 1s, this example simply duplicates the first three octets leaving the network ID of 200.20.16.

Table 4.4 illustrates the calculation that is performed.

Determining the network ID is very easy if the subnet mask is made up of only 255 and 0 values. Simply “mask” or cover up the part of the IP address corresponding to the 0 octet(s) of the subnet mask. For example, if the IP address is 15.6.100.1 and the subnet mask is 255.255.0.0, then the resulting network ID is 15.6.

For more complicated subnet masks, you can use the Windows Calculator in scientific view to convert between decimal and binary numbers, and use the “AND” button to perform a logical “AND.” For example, you can enter the number 240, select And, enter 35, and then select =. This gives you the decimal answer to the “AND.” You can then convert the result to binary if desired. Or you can enter the numbers in binary, converting the result to decimal when you are finished. However, you must use the same number system for both of the operands in the “AND” process when using the Windows Calculator.

You may have to use a subnet mask with values other than 255 and 0 if you need to subdivide your network ID into individual subnets. If you are not using subnets, you can use the default subnet mask that Microsoft TCP/IP assigns when configuring the IP address.

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