3.1 Switches, routers and ports

Previously we looked at IP addresses used by devices on a small domestic network, how each device needs an address, and that the format of these addresses is important so that they are allowed on the network.

The small domestic network that we’re looking at here is a very simple one. The chances are when you are using a network you will want to do something that requires connecting to devices on the other side of the gateway – somewhere in the cloud – a web server, for example. IP addresses are what enable us to do this, as all devices on the internet have an IP address.

There is another type of address: a MAC address. These also play an important role in delivering data, but across a local network rather than the internet.

In this session we’ll briefly look at how these two types of address are used and why both are necessary by exploring two very important networking terms: switching and routing. Both these terms are about getting data across networks from source to destination and have several points in common, so they are easy to confuse. We will then focus on switching here in more detail.

The home gateway device explored in earlier sessions – and like the one shown here – has several roles to play, including that of a switch and a router. A device like this one is designed to handle data traffic travelling to and from a number of devices that are typically found in the home.

The number of devices is likely to be higher than in the PT Anywhere domestic network we have been looking at. Many domestic networks will have dozens of connected devices, although most will be connected wirelessly. Even so, the number of devices is small when compared to an organisation that has their own network where there may be many computers connected to the network by cables or wireless connections. There may be different offices or buildings within the network to support.

For large-scale networks, routers and switches are separate devices, like these, and much bigger than domestic devices.

Routers and switches have several ports. In this context ports are sockets into which cables are plugged. The term ‘port’ is also used at other layers in the TCP/IP protocol suite, where they refer to specific applications that the data is associated with – so this can get confusing. But here we are talking about the physical sockets. Switches like these can have 24, 48 or more ports. Routers usually don’t have so many.

There are obviously a lot fewer ports on our domestic broadband device than on the larger switches and routers. These are designed to handle fewer devices and support more wireless, than wired, connections. This a typical example with four ports (in yellow) for connecting devices via cables. For example, you might want to connect a docking station for your computer, or a games console, as cable can provide faster connections.

I’ll now go back to our domestic network in Packet Tracer and take a quick look at the ports that are being used by the gateway. I’ll find out how many it has and which are connected. In Packet Tracer I can view the ports on the gateway device by clicking on it and going to the ‘Physical’ tab: this gives an impression of what the device would look like. The view here shows there are four ports in yellow for connecting devices. The blue port is for connecting to the internet. So a close representation of the home gateway device that we looked at previously that had four ports.

I can go to the ‘Options’ tab and ‘Preferences’ and then click the ‘Always Show Port Labels in Logical Workspace’ checkbox. When I do this, it starts to showing the ports that are connected at the gateway. We can see that PC0 is connected to port 2 on the router. Port 0 on the router is connected to the internet. According to our physical view that we looked at previously, this means we still have three spare ports on the gateway.