Lab Objective:
Apply VLSM to an existing network.
Lab Purpose:
Learn how to create subnets using VLSM.
Lab Tool:
Pen and paper.
Lab Topology:
N/A
Lab Walkthrough:
Your boss hands you network 172.16.0.0/16 and tells you that you need to apply VLSM to create two networks. The current topology is just one network which had the 172 network applied.
We know by now that we have one network, and with 16 bits available for hosts, we have 32766 hosts available for this subnet. This sort of addressing is used all over the world by lazy network engineers. Presumably, because private IP addressing space is free to use.
We now have to carve two subnets out of this network. Now, this isn’t a lesson on network design, which is a career path in its own rite. There are a number of ways to answer this problem, but I’ll just stick to what the boss asked for.
172.16.0.0 – 10101100.00010000.00000000.00000000
255.255.0.0 – 11111111.11111111.00000000.00000000 (original design)
Stealing one bit from the host’s bits and using it for subnetting gives us this:
172.16.0.0 – 10101100.00010000.00000000.00000000
255.255.128.0 – 11111111.11111111.10000000.00000000
172.16.128.0 – 10101100.00010000.10000000.00000000
255.255.128.0 – 11111111.11111111.00000000.00000000
We can effectively turn that stolen subnet bit on and off in the host address, giving us subnets 172.16.0.0 and 172.16.128.0.
Notes:
I don’t teach binary in this guide because it’s a lab book, but if you want some extra homework, then convert the binary back to decimal. It’s interesting to see how it all comes together. Check out my book, IP Subnetting – Zero to Guru, if you want to learn the theory.
