Variable Length Subnet Mask (VLSM) Example

Subnetting allows you to create multiple logical networks that exist within a single Class A/B/C network.

You are tasked to setup a network given a 172.29.0.0/16 address. We have 16 bits to work with for subnetting.

• 15 states
• 6 data centers in each state
• 50 servers per data center

Keep in mind we need point-to-point connections from HQ to the 15 regions and from the regions to each data center. That means we need at least 2 x 15 = 30 WAN IPs in the first subnet division.

So we need to borrow 3 bits from the 255.255.0.0 mask for a /19 mask with 32 hosts.

I’m not showing all the states but each network will be at increments of 32. Let’s take a look at the Colorado (172.29.96.0/19) network. There are 6 data centers. This means at least we need a subnet with at least (2×6) 12 hosts. Luckily we need a minimum of 50 hosts per subnet anyway. Depending on their growth intent (subnets vs more IPs). If the customer decides 50 clients are enough per data center, but they may add additional data centers, we should make sure we can accommodate the clients first. We’ll need to borrow 2 bits from the 4th octet (64 hosts), so our mask will be /26

The six data center networks:

Each center has a router that needs to be linked to the Colorado router. The point-to-point connections total to (5 x 2) 10 IPs. We can start from 172.29.97.128/26.

WAN IPs:

You can use this calculator to check:
http://www.vlsm-calc.net/