Decimal Algorithm for Deriving the Subnet Number Basic Subnetting

The decimal algorithm, when basic subnetting is in use, is as follows:

Step 1 Write down the IP address in decimal.

Step 2 Copy below the IP address either the first one, two, or three dotted decimal numbers of the address, based on whether the subnet mask is 255.0.0.0, 255.255.0.0, or 255.255.255.0, respectively.

Step 3 For the remaining dotted decimal numbers, record decimal value 0.

This algorithm is very similar to the algorithm that is used when there is no subnetting. The only difference is in Step 2. In fact, this later version of the algorithm would work fine when there is no subnetting in use. Table 5-15 lists several examples.

Table 5-15 Subnetting Examples

IP Address

Subnet Mask

Network and Subnet Part*

Full Subnet Number

8.1.4.5

255.255.0.0

8.1

8.1.0.0

130.4.100.1

255.255.255.0

130.4.100

130.4.100.0

199.1.1.4

255.255.255.0

199.1.1

199.1.1.0**

172.100.2.2

255.255.255.0

172.100.2

172.100.2.0

17.9.44.3

255.255.255.0

17.9.44

17.9.44.0

* The third column of Table 5-15 can be thought of as Step 2 in the decimal algorithm. ** This example shows a Class C address, with default mask. No subnetting is in use in this case.

* The third column of Table 5-15 can be thought of as Step 2 in the decimal algorithm. ** This example shows a Class C address, with default mask. No subnetting is in use in this case.

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