Looking Out To Sea

The internet protocol (IP) address of my laptop is 192.168.0.3/24. What does this mean?

The first and obvious question is, What’s an IP address? It’s an identifier that your computer uses to talk to other computers on the network. It bears a lot of resemblance in form and function to a telephone number. There are prefixes which are shared by every address in the area, and then there’s a bit specific to you.

An IP address has two important pieces of information embedded in it. The first is the host ID — the identifier of my specific computer. The second is the network address — which is the number of the network my computer is found in, and is analogous to a telephone area code.

Just to make things difficult though, the two numbers are joined together so you can’t tell where one part ends and the other begins. So when talking about addresses we need another piece of information to tell us which part is network and which part is host.

First, let’s write out 192.168.0.3 in binary. This makes a very long number but it will make everything much clearer from here on. Each number between the dots is converted separately. This number is not the same as 19,216,803.

192.168.0.3 = 11000000 10101000 00000000 00000011

I’ve left spaces where the dots were previously. Each section is 8 binary digits long, so each section can represent a maximum number of 11111111 — which is 255 in decimal.

The next bit we come to is choosing a point on that line so that all the digits on the left represent the network address and all the ones on the right show the host address. Looking back up at the address I gave at the top you’ll see a “/24” sitting at the end. This is called the network mask and it works just like a piece of card with a hole in it. You write out your address and then align 24 bits underneath: everything with a 0 underneath is masked out, leaving the network address. We are getting the result of 1 whenever both the address and the mask is 1.

192.168.0.3  = 11000000 10101000 00000000 00000011
mask 24 bits = 11111111 11111111 11111111 00000000
result       = 11000000 10101000 00000000 00000000

Back in decimal land, that network address is 192.168.0.0. Since the network mask is also a binary number it is often written like an IP address, as four decimal numbers separated by dots. The same address can be written as 192.168.0.3/24 or 192.168.0.3/255.255.255.0.

We can invert the network mask to give a host mask and use the same procedure to find out the host ID, which turns out to be 00000011, otherwise known as 3.

192.168.0.3 = 11000000 10101000 00000000 00000011
mask 8 bits = 00000000 00000000 00000000 11111111
result      = 00000000 00000000 00000000 00000011

You probably knew that from inspection but computers ain’t so clever!

Another useful number we can learn from the address and mask is the broadcast address. This can be used to send messages to everyone on the network. We calculate this by inverting the mask again, so we’ve got 8 bits on the right instead of 24 bits on the left, and taking the result to be 1 wherever the address or the mask is 1.

192.168.0.3 = 11000000 10101000 00000000 00000011
mask 8 bits = 00000000 00000000 00000000 11111111
result      = 11000000 10101000 00000000 11111111

The result can then be written as 192.168.0.255 in dotted-decimal format. Packets sent to this address will be examined by every host on the local network. This is useful if you don’t know the address of your recipient!

Photo is The World’s Network by saschaaa.