If you have an Apple Airport router, you should use their automated setup wizard. After using the setup process, you will need to make sure that it is in DHCP and NAT mode and not in OFF or (Bridge) mode.
To do that you need to use the Airport Utility on your computer and access your router. You can locate the Airport Utility in Finder, under Applications > Utilities.
In the utility, select your Airport router and click Edit.
Once you have logged in to your router, navigate to the Network tab and review the option: Router Mode. Be sure the selection says DHCP and NAT in the drop down (as displayed below).
Once you have done that, press the Update button at the bottom of the window. Then wait 2-3 minutes and your router should be set up properly to access our network.
At this point, you will likely be required to go through our login process one more time.
See the example here: