Every computer on the Internet has a unique identifier, its Internet address (IP address). The Internet protocol (IP) routes packets from one computer to another.

A router is a special-purpose computer which acts as an intermediary between a pair of communicating computers

An IP packet includes:

• The identity of the sender machine – i.e. it’s IP address
• The identity of the machine to which the packet should be delivered 
• The packet contents – application data

The maximum size permitted for an IP packet is 64Kb:

• In practice, this is too much for many networks to deliver in one chunk and the IP packet must be broken down into fragments
• The IP protocol takes care of disassembling a packet into fragments and subsequently reassembling the IP packet