2.1 Internet Protocol (IP) addressing

S2 Video 1 Introduction to IP addressing

In Session 1 you looked around a home network. You saw that it had a gateway device (sometimes called a router), and you saw that devices could be connected to the gateway via a wire or via Wi-Fi. You also saw how, at the command prompt of a computer, typing ipconfig revealed some of the network data of that computer. And you saw that typing pingand an IP number at the command prompt was a way to test whether there was an IP connection between two devices.

In this session I’m going to look further at the addressing system used in this kind of network. It’s called IP addressing, and the IP here stands for Internet Protocol. It’s the addressing system that’s used throughout the internet. You’ve already seen that a key part of this addressing system is the IP number, or the IP address as it’s often called.

I’m going to look at IP addresses by showing you some of the settings from a domestic gateway or router. The domestic gateway I’m going to use is part of this simulated network. This is the gateway here.

This simulates an actual product made by Linksys. The network also has a personal computer up here, which has a wired connection to the gateway, and it also has a tablet here with a Wi-Fi connection to the gateway. There’s also an internet connection here, just as with most other gateways. I’m using this simulation here because it makes it easier for me to show some of the things I want to talk about.

Domestic gateways like this usually have many settings that the user can change if they need to. Here’s a settings page of this gateway. There are quite a lot of settings there. I’ll just concentrate on these here. I’m going to pick out just a few because the rest are rather complicated.

To begin with, this number here 192.168.0.1 is the IP address of this gateway, or to be more exact it’s the IP address of the side of the gateway that’s connected to the local network. That’s because the gateway also has a connection to the internet. There’s a side that faces the internet, and that has a different IP address, and that’s up here: 99.0.0.2.

For the time being I want to concentrate on this network IP address here, 192.168.0.1. You might be wondering why the address consists of four numbers separated by decimal points. Why these particular numbers? Would other numbers work just as well? Other sessions will look at these questions in detail, but for the moment just accept that this is what an IP address looks like, or rather, it’s what the widely used version 4 of the internet protocol addressing system uses. There’s a version 6 of IP addressing that hasn’t been widely adopted for home networks and addresses look very different in IP version 6.

You’ll have spotted this number, this subnet mask, 255.255.255.0, which looks like an IP address, but actually it isn’t. I’ll explain what it’s for in a moment.

Now all the other devices on this network must have IP addresses, or they won’t be able to handle IP traffic. A network that can’t handle IP traffic, though, might be able to handlecarry other forms of traffic, such as Ethernet data. But to be able to cope with IP traffic, devices on a network must have IP addresses; and they must not only have IP addresses, they’ve got to have the right sort of IP address.

So what do I mean by ‘the right sort of IP address’? One thing I mean is that the address has to be right for the particular network. An IP address that works properly on one network probably won’t work on another. That’s because the IP address doesn’t just identify a device: it also identifies the network itself.

On this network, and on most home networks, the fourth number of the IP address identifies the device. The three numbers before it identify the network. This zero in the subnet mask shows which part of the IP address labels the device. These 255s in the subnet mask show the network parts of the IP address. For example, this subnet mask on a network shows that the last two numbers of an IP address on the network are for the device, and the first two numbers are for the network itself. But this would be very unusual on a home network.

Returning to our home network, the gateway here has a 1 as the last of the four numbers in its IP address. Because the subnet mask is 255.255.255.0, that 1 in the gateway’s IP address identifies the device, the gateway.

Only one device on the network can have the number 1 here in the fourth place. And only one device could have the number 2, or the number 3 and so on. It’s similar to the way on a street only one house can be number 1, and only one can be number 2, and so on. You need unique numbers to be able to get things delivered to the right place. A gateway doesn’t have to have the number 1, but it often does.

So here’s a rule of IP addressing: The part of the IP address that identifies the device must be unique on the network. You can’t have more thant one device with that number.

On this network, the first three numbers of an address identify the network. So in the case of the gateway, the numbers 192.168.0 identify the network. Every device on this network must have these same numbers as the first three numbers of its IP address. These three numbers are a bit like the town and street of a postal address, in the sense that every house on a street must have the same street name and town in its address.

So here’s another rule for IP addressing. The network part of the address must be the same for all devices on the network.

And just to close this part, another rule. The subnet mask has to be the same for every device on the network.