Step 3: Define the Subnet Mask

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Step 3: Define the Subnet Mask

The next step is to define for the entire internetwork one subnet mask that gives the desired number of subnets and allows enough hosts per subnet.

As shown previously, the network ID of an IP address is determined by the “1s” of the subnet mask, shown in binary notation. To increase the number of network IDs, you need to add more bits to the subnet mask.

For example, you are assigned a Class B network ID of 168.20.0.0 by InterNIC. Using the default Class B subnet mask 255.255.0.0, you have one network ID (168.20.0.0) and about 65,000 valid host IDs (168.20.1.1 through 168.20.255.254). Suppose you want to subdivide the network into 4 subnets.

First, consider the host 168.20.16.1, using the subnet mask 255.255.0.0. In binary notation, it is represented as shown in Table 4.7.

Remember, the subnet mask 1 bits correspond to the network ID bit in the IP address.

By adding additional bits to the subnet mask, you increase the bits available for the network ID and thus create a few more combinations of network IDs.

Suppose that in the example in Table 4.7 you add three bits to the subnet mask. The result increases the number of bits defining the network ID and decreases the number of bits that define the host ID. Thus, you have more network IDs, but fewer hosts on each subnet. The new subnet mask is:

Subnet Mask: 11111111 11111111 11100000 00000000

As you have three extra bits in the network ID, you now have six different network IDs. All 0s or all 1s are not allowed, because these are reserved for the broadcast-type addresses. All 0s mean “this network only,” and all 1s mean broadcast. Table 4.8 shows all the possible subnet IDs using the network ID of 168.20.0.0 with a subnet mask of 255.255.224.0.

Note that if you use only two additional bits in the subnet mask, you are only able to have two subnets. The network IDs that result in Table 4.8 are as follows:

10101000.00010100.01	168.20.64.0
10101000.00010100.10	168.20.128.0

Therefore, you must use enough additional bits in the new subnet mask to create the desired number of subnets while still allowing for enough hosts on each subnet.

After you determine the number of subnets you need to create, calculate the required subnet mask as follows:

1.	Add 1 to the number of subnets needed and convert the result to binary format. (Like the host ID, the subnet ID cannot be all 0s or all 1s—adding 1 avoids these possibilities.) You may want to use the Windows Calculator in Scientific view.
2.	The number of bits you used to write the required subnets in binary is the number of additional bits that you add to the default subnet mask. You also need to include any 0 bits in the count. For example, if you need eight subnets that is the binary number 1000. This means you need four binary digits or bits in the subnet mask.
3.	Place the number of binary digits needed at the beginning of the octet, and then fill the remaining eight digits in the octet with 0s.
4.	Convert the subnet mask back to decimal format. This value replaces the first 0 octet in the subnet mask.

Suppose, for example, that you are assigned a Class B network ID of 168.20, and you need to create 5 subnets. Following the preceding steps, converting 5 into binary gives 00000101, or simply 101.

This means you need three bits to give enough combinations for 5 networks. Therefore, you need to add three bits to the default subnet mask. The default subnet mask for a Class B network is 255.255.0.0, or in binary is:

Default subnet mask

11111111.11111111.00000000.00000000

Adding the three bits creates the custom subnet mask:

Custom subnet mask

11111111.11111111.11100000.00000000

If you convert this to decimal, you will see the subnet mask is 255.255.224.0.

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