1) That blue or green network cable coming out of your computer/modem/router is actually a busy highway comprised of 65,536 tiny electronic lanes (yes, over sixty-five thousand little lanes for your electrons). Each lane is called a "port", and each port is designed to allow only specific types of information through. Many ports are assigned in a semi-standardized way. Here are some example port assignments:

• HTML pages: port 80
• FTP file transferring: port 21
• World of Warcraft: port 3724
• POP3 email: port 110
• MSN Messenger: port 6901 and ports 6891-6900
• Everquest: port 1024
• Bit Torrents: port 6881

2) Bit torrents, by default, first look for port 6881 to enter/exit your computer. If port 6881 is closed for whatever reason, torrents then go to port 6882, then 6883, and so on, until it eventually tries port 6999 before giving up.