CIDR: what the /24 means

How long is the street name?

Picture a long road with two different signs. On one road, the street is called Oak and the houses are numbered 1 to 250. On another, the street is called Oak Avenue North and there are only 6 houses. The longer the street’s name, the fewer houses fit on it.

In lesson 3.1 you saw that an IP address is a network part (the street) and a host part (the house number). But 192.168.1.10 on its own doesn’t say where one ends and the other starts. That’s what the number after a slash is for:

192.168.1.10/24

The /24 is the prefix length. It means the first 24 bits are the network part. An address has 32 bits, so the other 32 − 24 = 8 bits are the host part.

in one sentence The number after the slash says how many bits, from the left, are the network part.

CIDR: just the slash

Writing networks as an address, a slash and a number is called CIDR (say cider). It stands for Classless Inter-Domain Routing. That’s a long name for a simple idea: say exactly how long the network part is, with a slash. (Long ago, networks only came in three fixed sizes called classes, which wasted lots of addresses. Classless means any size you like.)

in one sentence CIDR is the way of writing a network with a slash, like 192.168.1.0/24.

The subnet mask

There’s a second way of writing the same thing, which you’ll see on computers and phones: the subnet mask. Think of a stencil laid over the address: the holes show the network part, and the solid bits cover the host part.

To make it, write a 1 for every network bit and a 0 for every host bit, then turn each group of 8 into a number, like the light switches in lesson 3.1:

/24  =  11111111.11111111.11111111.00000000  =  255.255.255.0

So 192.168.1.10/24 and 192.168.1.10 with mask 255.255.255.0 mean exactly the same thing.

in one sentence The subnet mask, like 255.255.255.0, says the same thing as the slash, written as four numbers.

Two addresses nobody can have

Every network keeps two of its addresses for special jobs:

• the network address: the very first one, with every host bit 0. It’s the name of the whole network, like the name of the street, for example 192.168.1.0.
• the broadcast address: the very last one, with every host bit 1. Send a packet to it and everyone on the network gets it (the broadcast from lesson 2.2), for example 192.168.1.255.

So the hosts get everything in between.

in one sentence The first address names the network and the last one means everyone, so hosts get the ones in between.

How many hosts fit?

Every extra host bit doubles the number of addresses: 1 bit gives 2, 2 bits give 4, 3 bits give 8, and so on. Mathematicians write that as 2 to the power of the number of bits: 2⁸ = 256. Take away the two special addresses, and you’ve got the number of hosts:

Notice: every time the prefix goes up by one, the network is cut in half. Longer street name, fewer houses.

in one sentence Hosts = 2 to the power of the host bits, minus 2; each step bigger in the prefix halves the network.

Why a host cares

Before sending anything, a host asks itself: is this address on my network? It compares the network part of its own address with the network part of the address it’s sending to. Same street? Send it straight there. Different street? Send it to the default gateway (the router, the way out, from lesson 2.1).

in one sentence Same network part: send it straight there; different: send it to the gateway.