IP
The Internet Protocol, or IP, is a very basic form of message addressing. At its heart, IP is nothing more than an envelope in which another packet (normally a TCP or UDP packet) travels to arrive at its destination.
Think of the IP as the envelope that you use when sending a letter. A letter has a recipient address and a sender or return address. IP packets contain this same information and a little more. The IP packet is basically just a header stuck on another (usually TCP) packet with a destination and source address. It also contains a checksum, an option field, and a few other fields. The exact format and contents can be found in RFC 791, available from http://www.rfc-editor.org/rfc.html.
When you want to send a letter to Barney ...
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