Defining the Network IDs Manually

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Defining the Network IDs Manually

The network ID for each subnet is determined using the same number of bits as were added to the default subnet mask in the previous step. Use the following steps to define each subnet network ID:

1.	List all possible binary combinations of the additional bits added to the default subnet mask.
2.	Discard the combinations with all 1s or all 0s. All 1s or all 0s are not valid as network IDs, because all 1s represents the broadcast address for the subnet and all 0s implies “this network only” as a destination.
3.	Convert the remaining values to decimal notation. Remember you must use the full 8 bits of the octet for the binary number that is converted to decimal.
4.	Finally, each value is appended to the original assigned network ID to produce a subnet network ID.

If you were assigned a Class B network ID of 168.20.0.0 and need to create at least 5 subnets. You need an additional 3 bits added to the default subnet mask to create the subnets. The new subnet mask is then 255.255.224.0, or in binary:

11111111 11111111 11100000 00000000

Listing all combinations of the additional bits gives the following:

000

001

010

011

100

101

110

111

Discarding the values 000 and 111, and converting the remaining combinations to decimal format, you have the following:

32.	00100000
64.	01000000
96.	01100000
128.	10000000
160.	10100000
192.	11000000

Appending the preceding values to the original assigned network ID gives the following new subnet network IDs:

168.20.32.0

168.20.64.0

168.20.96.0

168.20.128.0

168.20.160.0

168.20.192.0

All the new subnet network IDs use the subnet mask of 255.255.224.0.

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