Lesson 5 of 7
Splitting a Block With VLSM
Variable length subnet masking (VLSM) lets one block hold subnets of different sizes. The rule that keeps it tidy: allocate the largest first.
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Equal Splits First
Borrowing bits splits a block into equal parts. Going from /24 to /26 borrows 2 bits, so 22 = 4 subnets of 64 addresses each:
10.53.10.0/26
,
10.53.10.64/26
,
10.53.10.128/26
,
10.53.10.192/26
.
Each subnet starts where the last one ended. Subnet number n starts n − 1 blocks in.
Worked Example: One /24, Four Needs
We have 10.53.10.0/24 and four groups to address. Sort them largest first, give each the smallest
prefix that fits (lesson four), and place each at the next free address:
| Need | Hosts | Subnet | Usable Range |
|---|---|---|---|
| Staff | 100 |
10.53.10.0/25
|
10.53.10.1 to 10.53.10.126
|
| Wi-Fi guests | 50 |
10.53.10.128/26
|
10.53.10.129 to 10.53.10.190
|
| Servers | 20 |
10.53.10.192/27
|
10.53.10.193 to 10.53.10.222
|
| Link to the firewall | 2 |
10.53.10.224/30
|
10.53.10.225 to 10.53.10.226
|
That leaves 10.53.10.228 to 10.53.10.255 free for later.
Why Largest First
A block can only start on a multiple of its own size. Place the small subnets first and the large one may not find a properly aligned gap, even when the free addresses add up. Largest first never has that problem.
Practice
5 questions, new each time. Every answer is worked out by the same library as our subnet calculator, so if one looks wrong, check your working first and then tell us.