But when behind no routers and connected directly to a ADSL modem, you'll need to do some configuration before you can get access to the Internet. Run,
$ sudo pppoeconf
and follow the instructions on-screen. When in doubt, stick to the defaults.
Once everything's set up, just do:
$ sudo pon dsl-provider
to connect, and:
$ sudo poff
to disconnect. Simple as that ;-)