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Transmissions over a network can be extremely large in size. To send a large message in one go would be impractical, as both the sending and receiving node would be tied up with one communication.

Instead, a method called packet switching is used. With packet switching, messages are broken up into very small pieces, called packets. Each packet consists of two parts:

• header - this includes the sender's and recipient's IP addresses, the packet number, the total number of packets the message contains, plus the details of any protocols used
• payload - this is part of the actual message itself

The packets are sent individually across the network and put back together to reform the message at the other end.

Packets may or may not follow the same route - they are sent along whichever route allows the quickest transmission. Packets, therefore, may arrive out of sequence. The recipient takes each packet, makes a note of its number and assembles them into the correct order.

Sending small packets allows a node to send more than one message at the same time. It is rather like speaking to two people at the same time, saying one word to each person in turn.

Packet switching also helps to ensure messages arrive complete without slowing down a network. If the recipient finds packets are missing, it can request that just those packets are re-sent. This saves having to resend the whole message, which is especially useful if the message is a large video file.