Do you have a Windows computer running? If so there should be a CD that
configures your DSL modem, maybe router. Is it a "2Wire modem"? The thing
here is to configure it using the Windows box and the CD. It sounds like it
might already be ready.

You might want to check the configuration of the DSL modem and see if it
serves up DHCP, if it does then all you have to do is enable DHCP on your Ubuntu
box and you're ready.

I'll be back in about 30 minutes, grandson over and dinner on table.