Router

A router links two or more separate networks together and forwards data between them. You use one to join a LAN to another network, such as the internet, so devices on your network can reach devices that are not on it. A router does not link individual devices together inside a LAN in the way a switch does.

How a router handles packets

Data sent across networks is split into packets. A router receives each packet, reads the destination address and forwards the packet towards the network it is addressed to. It keeps a routing table: a list of networks it knows about and which connection to send data out through.

A routing table can hold more than one possible route to the same network, so the router can identify an efficient path, such as the one with the fewest links or the least busy connection. Packets then arrive sooner and crowded links are avoided.

Other tasks

A router can assign IP addresses to devices joining its network, so each device has its own address and replies can find their way back to that device. It can also convert packets between protocols: a packet arrives following one set of rules and leaves following the rules the next network expects, so networks following different rules can still exchange data.

The internet

The internet is not one network but many networks joined by routers. A packet is passed from router to router, each one reading the destination and choosing where to send it next, until it reaches the network holding the destination device.

Example: a museum has one network in its main gallery and another in the ticket office across the car park. A router joins the two, so a till can look up an item held on the gallery's collections computer. The router reads each packet's destination, checks its routing table and forwards the packet down the correct link.