What is the IP address of your router?
You need to ensure that the first 3 octets are the same as utri007 says if your Amiga has a netmask of 255.255.255.0 and also no other machine on your network is using the chosen address. (Its best to ping that address from another machine with the Amiga not connected to make sure)
You need to set this router address in the gateway field, and also in the DNS server address if your router supports DNS forwarding (99% of modern routers do).
Hope this helps, Gaz