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The IP Protocol

Recall earlier: the TCP (transport layer) protocol is implemented in the two end systems involved in communications -- engineers say it's implemented at the "edges" of the network.
 
TCP relies on the Internet Protocol to deliver packets of data from one end system to another. The packets which IP delivers are usually called datagrams.
 
Datagrams navigate zero or more (sometimes many) routers, interconnecting the individual networks which together make up the Internet. The word "Internet" is just a contraction of the phrase "Interconnected Networks".
 
A router is essentially a special-purpose computer whose sole function is to transfer datagrams between networks. In earlier times, they were called "IP Gateways". The general structure of the Internet can thus be visualised:
Router-connected networks


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