VLSM Subnet Planner

Understanding the VLSM subnet planner — concept and principle

VLSM (variable-length subnet masking) means splitting one address block into subnets of different sizes, each sized to its own need, instead of cutting it into equal pieces. The planner takes a base network and a list of required host counts and returns a non-overlapping network address and prefix for each subnet. It is used when designing office, branch or lab networks where a user LAN needs 100 addresses but a router-to-router link needs only 2.

How it is calculated

1. Sort the requests from largest to smallest.
2. For each request h, find the smallest number of host bits b with 2^b ≥ h + 2 (two extra addresses for network and broadcast): b = ⌈log2(h + 2)⌉. The prefix is 32 − b and the block size is 2^b.
3. Round the current address up to the next multiple of the block size so the subnet is aligned.
4. Assign that block, then move the pointer past it and continue.

Results are labelled with the original request number (#1, #2 …) so you can match them to your list.

The principle

A subnet of size 2^b must start on an address divisible by 2^b, otherwise its mask would cut through the middle. Placing the biggest blocks first keeps every later, smaller block naturally aligned and leaves no gaps, which is why largest-first is the standard VLSM method. Using the smallest prefix that fits each request avoids wasting addresses on oversized subnets.

Worked example

1. Base 10.0.0.0, requests 100, 50, 20 and 2 hosts (already in descending order).
2. 100 + 2 = 102 → 2^7 = 128, so /25. Assigned 10.0.0.0/25 (10.0.0.0–10.0.0.127).
3. 50 + 2 = 52 → 2^6 = 64, so /26. Next aligned start is 10.0.0.128: 10.0.0.128/26.
4. 20 + 2 = 22 → 2^5 = 32, so /27 at 10.0.0.192/27.
5. 2 + 2 = 4 → 2^2 = 4, so /30 at 10.0.0.224/30.
6. The plan uses 10.0.0.0 to 10.0.0.227; the rest of 10.0.0.0/24 is free for growth.

How to read the result

Each line gives the network address and prefix; usable hosts are 2^(32 − prefix) − 2, so a /25 holds 126, a /26 62, a /27 30 and a /30 2. Spare capacity inside each subnet is the difference from your request. Check that the last subnet still lies inside the block you actually own.

Common mistakes

• Forgetting the 2 reserved addresses: 62 hosts fit a /26, but 63 need a /25.
• Allocating small subnets first, which creates alignment gaps and wasted space.
• Not leaving room for growth; it is often wiser to round up one size.

Frequently asked questions

Why is the largest subnet allocated first?

Large blocks have the strictest alignment; placing them first lets smaller ones fill the remaining space without gaps.

What prefix fits 30 hosts?

A /27, which provides exactly 30 usable addresses.

Should point-to-point links use /30 or /31?

Both work. /30 is universal; /31 saves addresses where both devices support RFC 3021.

Sources: IETF RFC 1878, Variable Length Subnet Table for IPv4 · IETF RFC 4632, Classless Inter-domain Routing (CIDR)