Contrary to many people's beliefs, it is very difficult, almost impossible to completely mask/hide your ip while connected to the internet. As Freddy stated earlier in this post, a computer cannot communicate on the Internet without knowing the destination computer's IP address and the destination computer has to have the IP address of your computer. This goes for any computer communicating via the TCP/IP protocol.
If you want to find out more information about what an IP address is or what can be done with an IP address I would suggest doing a search, which would bring up lots of information.
Here is a few sites that I brought up on the subject:
this site will show you your ip address, or the address that others will see (if you are going through a remote proxy)
http://www.whatismyip.com/
For those somewhat technically savy here is a more technical explanation of TCP/IP addressing and networking.
http://compnetworking.about.com/libr.../aa042400a.htm
for those not so technically savvy here is a page with an explanation in layman's terms.
http://www.usd.edu/trio/tut/start/ipaddress.shtml