Subnet a /24 in your head in under a minute

Students meet subnetting as binary arithmetic, get it wrong under exam pressure, and conclude they are bad at networking. Nothing about it is hard once you stop converting to binary and start counting in blocks.

One table, memorised once

Every subnet mask longer than /24 corresponds to a block size — how far apart the network addresses sit in the last octet. There are only six worth knowing:

/25  mask 255.255.255.128   block 128   126 usable hosts
/26  mask 255.255.255.192   block  64    62 usable hosts
/27  mask 255.255.255.224   block  32    30 usable hosts
/28  mask 255.255.255.240   block  16    14 usable hosts
/29  mask 255.255.255.248   block   8     6 usable hosts
/30  mask 255.255.255.252   block   4     2 usable hosts

The block size is 256 − last octet of the mask, and the usable host count is the block size minus two — one for the network address, one for the broadcast. That is the whole method. Everything below is just applying it.

If you can say the block size out loud the moment you see the prefix, every exam question becomes a counting exercise.

Worked example 1: which subnet is this host in?

Question. A host is configured as 192.168.10.100/26. What are its network address, broadcast address and usable range?

Work. A /26 has a block of 64, so the subnets start at 0, 64, 128 and 192. The host is 100, which falls between 64 and 128. So the network is 192.168.10.64, the next subnet starts at 128, and the broadcast is one below that: 192.168.10.127.

• Network: 192.168.10.64
• Usable: 192.168.10.65 – 192.168.10.126
• Broadcast: 192.168.10.127

Worked example 2: pick a mask for a given host count

Question. A department needs 30 usable addresses. What mask do you give it?

Work. Read the table backwards. 30 usable means a block of 32, which is a /27 — 255.255.255.224. A /28 would only give 14, and a /26 would waste 32 addresses. Note the trap: if they had asked for 31 hosts, /27 is no longer enough and you move up to /26.

Worked example 3: a different octet

Question. 172.16.5.200/28 — what is the broadcast address?

Work. Block of 16. The multiples of 16 below 200 are 192; the next is 208. So the network is 172.16.5.192 and the broadcast is 172.16.5.207. The first three octets never move, because the prefix is longer than /24.

Worked example 4: how many subnets?

Question. How many /30 links can you carve out of a single /24?

Work. 256 addresses divided by a block of 4 is 64 subnets, each with two usable addresses. That is why /30 is the standard mask for a point-to-point link between two routers — exactly enough for both ends and nothing spare.

How to drill it

1. Write the six-row table from memory every morning for a week. It takes forty seconds.
2. Then do ten random questions a day with a timer. Aim for under fifteen seconds each.
3. Once the arithmetic is automatic, configure it. Put the addressing on real interfaces, ping across the subnet boundary, and watch it fail when the mask is wrong.

The third step is the one people skip, and it is the one that makes the first two stick. We run that drill in the lab during NET-201 (CCNA 200-301), on the switches rather than on paper.