Simple Answer is google it... i got this answer from answers.yahoo.com
Best Answer - Chosen by Voters
I'm betting that your computer is using DHCP (Automatically get an IP address). You turned it on and it got an IP address that's different than the address it had when you forwarded the ports.
Assign that computer a static address (see http://www.portforward.com/networking/staticip.htm and choose your operating system), forward the ports to that address, and you shouldn't have the problem again.
http://answers.yahoo.com/question/index?qid=20090705181222AAhAlmL