Subnetting with VLSM

Sharing a pizza fairly

Five friends share a big pizza. One is starving, one is quite hungry, and three just want a nibble. If you cut it into five equal slices, the starving one goes hungry and the nibblers leave crusts. If you cut big slices for the hungry ones and small slivers for the rest, everyone gets what they need and there might even be some left over.

in one sentence Give each person the size of slice they need, not the same slice for everyone.

Subnets

A school or company is usually given one block of addresses, like 192.168.10.0/24 (256 addresses: from lesson 3.2). Cutting that block into smaller networks is called subnetting, and each small network is a subnet (sub means under or part of, like a submarine goes under the sea).

Why cut it up?

• Groups: students, teachers and cameras each get their own subnet, so each can have its own rules.
• Fewer announcements: each subnet is its own broadcast domain (lesson 2.2), so broadcasts stay smaller.
• No waste: each group gets the size it needs.

in one sentence Subnetting cuts one block of addresses into smaller networks called subnets.

VLSM: a different size for each

The simple way is to cut equal pieces, but groups aren’t equal: 100 students and 2 addresses for a cable between two routers. Equal pieces big enough for the biggest group would waste nearly all of the small ones.

VLSM stands for Variable Length Subnet Masks. Variable means can change, and the length is the prefix from lesson 3.2. So: each subnet gets its own prefix, just big enough for its group. That’s the pizza cut to fit.

in one sentence VLSM gives each subnet its own prefix, so each group gets the size it needs.

Sizes come in doubles

Subnets can’t be any size: they come in powers of two, numbers you get by doubling, starting from 1: 2, 4, 8, 16, 32, 64, 128, 256. That’s because each host bit doubles the addresses (lesson 3.2). And remember, every subnet loses two addresses: the network address and the broadcast.

in one sentence A subnet’s size is a power of two, and two of its addresses are always taken.

The recipe

1. List the groups, biggest first.
2. For each group, add 2 (for the network address and the broadcast).
3. Find the smallest power of two that’s at least that number. That’s the block size. Which power it is, is the number of host bits: 64 = 2⁶, so 6 host bits.
4. The prefix is 32 minus the host bits: 32 − 6 = /26.
5. Place it at the next free address. A block has to start at a multiple of its own size (a block of 64 starts at 0, 64, 128 or 192). Going biggest first means it always does, with no gaps.
6. Write down the network (the first address), the hosts (everything between) and the broadcast (the network plus the block size, minus 1).

in one sentence Biggest first, add 2, round up to a power of two, place it at the next free address.