VLSM Design Drill #1

VLSM causes many #CCENT and #CCNA candidates much heartache. Today’s post shows a VLSM design exercise that is useful for exam prep as well as real life. The idea: begin with an existing design that happens to use VLSM. Then plan the next subnet to add to the network. Can you find a subnet number to use that doesn’t overlap with the existing subnets? Today’s post lists the question; next time, the answer. Details below the fold.

The Scenario

Your boss wants you to add a subnet to an existing design. The existing design already has these five subnets:

1. 192.168.1.32/29
2. 192.168.1.0/28
3. 192.168.1.128/29
4. 192.168.1.96/30
5. 192.168.1.192/27

The boss cannot decide among three competing subnet masks. However, while he’s thinking about which mask to use, he wants you to practice VLSM and choose the subnet ID he would use for each of those three possible masks. He tells you that the new subnet ID must be part of class C network 192.168.1.0, that the new subnet must not overlap with the original five subnets, and that the new subnet ID must be the numerically lowest possible subnet ID (without breaking the other rules).

The following list details the prefix masks your boss is considering using for the new subnet. Choose the subnet ID you would recommend in each case.

1) /26

2) /27

3) /28