1. On your router in the Basic Setup section, check to see if the DHCP Server is set to Enable. This is the most common configuration for a home network. If it is, then you should have all network settings on the XBox 360 set to <b>Automatic</b>. This allows the XBox to get all information for IP/DNS/Subnet Mask from the router. There shouldn't be a reason to have to enter these in automatically unless your DHCP is disabled.
2. Swap the ethernet cable with another. Cables sometimes do fail, or you may be inadvertently using a crossover ethernet cable.
3. Try a different router.
4. Find a tech savvy friend to help you.
Hope this helps.