4. Router

A Router is a device that transfers data from one network to another in an intelligent way. It has the task of forwarding data packets to their destination by the most efficient route.

Above: A domestic combined router / ADSL modem

In order to do this, the router has a micro computer inside it. This holds a table in memory that contains a list of all the networks it is connected to, along with the latest information on how busy each path in the network is, at that moment. This is called the 'routing table'.

When a data packet arrives, the router does the following:-

- Reads the data packet's destination address

- Looks up all the paths it has available to get to that address

- Checks on how busy each path is at the moment

- Sends the packet along the least congested (fastest) path

Other tasks the Router can perform:

- Exchange protocol information across networks

- Filter traffic - helps prevent unauthorised intrusion by malware

Routers are also needed to enable a computer to connect to the internet, after all, the internet is just one vast external network.

If the data packet has a destination address outside the local networks, then the router may send it to the internet modem and then on to the ISP's router at the other end of the line. Their router will then pass forward the data packet towards its destination. For a computer say in the UK to connect to a web server in the USA the data packet will pass through many routers around the world.

Hopping around the world

Just to show you real routers in action on the internet, I ran a program called 'tracert' on my computer to connect to a web site in the USA. Tracert shows the 'hops' the data packet takes as it is passed from router to router and finally making contact with the web server in the States.

You can see the packet goes to a router in Birmingham (Hop 9), passes through a router in Chicago (Hop 20), then Denver and bounces to a router in Seattle (hop 25) and finally gets to the web server.