A Shortcut for Defining the Network IDs

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A Shortcut for Defining the Network IDs

Defining the network IDs manually becomes tedious when more than three additional bits are added to the default subnet mask, because it requires listing and converting many bit combinations. The following is a shortcut method for defining the subnet network IDs:

1.	After determining the new subnet mask you calculated for the required number of subnets and host IDs, list the additional octet added to the default subnet mask in decimal notation.
2.	Convert the rightmost 1-bit of this value to decimal notation. This is the lowest order 1-bit in the octet you calculated. This decimal value is the incremental value between each subnet value, known as “Delta.”
3.	The maximum number of subnet network IDs that can be used with this subnet is 2 less than 2 to the power of n, where n is the number of bits you determined were needed for your subnet (# of subnets = (2^n)-2).
4.	Append “Delta” as an octet to the original network ID to give the first subnet network ID.
5.	Repeat Step 4 for each subnet network ID, incrementing each successive value by “Delta.”

Again if you are assigned a Class B network ID of 168.20.0.0 and need to create at least 5 subnets. You needed an additional 3 bits added to the default subnet mask to create the subnets.

The additional bits added to the default subnet mask are 11100000.

The rightmost bit converted to decimal (00100000) is 32. Thus, the incremental value is 32. There will be (2^3)-2 =6 subnets created.

The subnets created are as follows:

168.20.0.0 and 32 = 168.20.32.0

168.20.32.0 and 32 = 168.20.64.0

168.20.64.0 and 32 = 168.20.96.0

168.20.96.0 and 32 = 168.20.128.0

168.20.128.0 and 32 = 168.20.160.0

168.20.160.0 and 32 = 168.20.192.0

If you increment the last subnet network ID once more, the last octet matches the last octet of the subnet mask (224), which is considered a broadcast address and thus is an invalid network ID.

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