IP stands for "Internet Protocol". It can be thought of as the common language of computers on the Internet. There are a number of detailed descriptions of IP given elsewhere, so we won't cover it in detail in this document. However, it is important to know a few things about IP in order to understand how to secure your computer. Here we’ll cover IP addresses, static vs. dynamic addressing, NAT, and TCP and UDP Ports.
An overview of TCP/IP can be found in the TCP/IP Frequently Asked Questions (FAQ) at
http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part1/
and
http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part2/

source:- http://www.cert.org/tech_tips/home_networks.html