It has to do with how a router is designed to work when you have an open nat that means the ports Xbox Live uses are open.
I looked up your router and the model number I found is WRT310N please look at the router you purchased and confirm.
Steps to follow:
1) Open a browser.
2) type 192.168.2.1 in your address bar and press enter.
3) type in admin for username and password for password.(this should be it if this doesn't work try again but leave your password blank.)
4) Click the Applications & Gaming link near the top of the page.
5) Click Port Range Forwarding link.
6) Type in Xbox Live for application name, make the first port 53 and the next port 3074.
7) Turn on your Xbox 360 go to your network settings and find out what IP address your Xbox 360 is using(only need the last number.)
8) Put the last number from the IP address that your Xbox 360 is using and type it under IP address.
9) Make sure enabled is checked.
10) save your settings and your done.
http://portforward.com/english/routers/port_forwarding/Linksys/WRT310N/Xbox_Live_360.htm If you still have problems with your friends make sure they have forwarded their ports also.
<message edited by FordGTGuy on Saturday, October 17, 2009 8:55 PM>