SubnetKit › VLSM Calculator for CCNA
VLSM Calculator for CCNA: how it works
Variable Length Subnet Masking lets you carve one address block into subnets of different sizes so that a 60-user LAN and a 2-address WAN link do not each burn a /24. The CCNA tests the method, not just the answer: allocate largest first, align each block to its size and avoid overlap.
SubnetKit's VLSM tab does exactly that in the browser, free, and shows every intermediate value so you can check your own working against it.
Worked example: four networks from 192.168.10.0/24
Requirements: Sales 60 hosts, Engineering 28, Admin 12, WAN link 2. Sorted largest first:
- Sales, 60 hosts → /26 (64 addresses, 62 usable): 192.168.10.0/26, mask 255.255.255.192, hosts .1–.62, broadcast .63
- Engineering, 28 → /27 (32, 30 usable): 192.168.10.64/27, mask 255.255.255.224, hosts .65–.94, broadcast .95
- Admin, 12 → /28 (16, 14 usable): 192.168.10.96/28, mask 255.255.255.240, hosts .97–.110, broadcast .111
- WAN link, 2 → /30 (4, 2 usable): 192.168.10.112/30, mask 255.255.255.252, hosts .113–.114, broadcast .115
Total used: 116 of 256 addresses; the next free address is 192.168.10.116, leaving room for growth. These rows are preloaded in the calculator.
How to pick each prefix
For a requirement of n hosts, find the smallest power of two that is at least n + 2 (network and broadcast), then the prefix is 32 minus that exponent. For 28 hosts: 28 + 2 = 30, the next power of two is 32 = 2^5, so the prefix is 32 − 5 = /27.
Two rules trip people up. First, each block must start on a multiple of its own size: after the /26 ends at .63, a /27 can start at .64 because 64 is divisible by 32. Second, allocate in descending order of size; placing the WAN /30 first at .0 would force the /26 to start at .64 and waste the gap. SubnetKit reports the wasted addresses per row so you can see the cost of any plan.
Frequently asked questions
What is VLSM in simple terms?
Using different subnet masks within one network so each subnet is sized to its real host count, instead of giving every segment the same fixed size.
Why allocate the largest subnet first?
Large blocks have strict alignment (a /26 must start on a multiple of 64). Placing them first keeps the blocks contiguous and prevents gaps that small subnets could otherwise have filled.
Does the calculator support /31 for point-to-point links?
Yes. Toggle the /31 option and any requirement of 2 hosts becomes a /31 per RFC 3021, and a requirement of 1 becomes a /32.
How many networks can I plan for free?
Up to 5 requirements per plan on the free tier, which covers typical CCNA practice questions. Pro removes the limit for larger designs.
Looking for the full guide, tips and the complete FAQ? Read the main SubnetKit page.