A: Your IP (Internet Protocol) address is your computer's unique identification on the internet or local network.
A: In Windows, click Start»Run, enter cmd and type ipconfig.
A: Your address on the internet is assigned by your ISP. Most broadband routers use NAT (Network Address Translation) to reroute data to your local network address, so your computer doesn't actually know the address by which it is identified on the internet. If you have a number of computers sharing an internet connection, they share the same internet IP address. One way to find out that address is to point your browser at http://www.whatismyip.com
A: In general, your ISP assigns you a Dynamic IP address, which means it will change from day to day or week to week (or on each call for a dial-up connection). However you can get a static IP address on broadband by paying a bit extra.
A: The subnet mask restricts the range of IP addresses in a network. A '1' bit in the subnet mask means the corresponding bits in the IP addresses of two computers must be the same to allow them to exchange data. Thus if the subnet mask is 255.255.255.0, and my computer's address is 192.168.16.1, it can only communicate with IP addresses 192.168.16.xxx. This mechanism allows a number of logically separate subnetworks to share the same ethernet media.
A: There are three groups of IP addresses reserved for private networks. ISPs program their routers so that packets with these addresses are not forwarded through the internet. When setting up a local network, you should use numbers from these ranges:
See http://www.rfc-editor.org/rfc/rfc3330.txt for more details.