My Ubuntu Server (12.04) box doesn't ping my ADSL modem. I've setup a simple network which connects a XP box, a Ubuntu Server box and an ADSL modem. The XP box pings the Ubuntu Server and the ADSL modem, but the Ubuntu Server pings only the XP box. "Destination host unreachable" is reported whenever the Ubuntu Server pings the ADSL modem.
How should I tackle the problem?