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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:

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