If you cannot ping a host then try some of these (commands included with -...-):
1) Can you ping your own Ethernetcard/IP address?
2) Look at your routing table with -route-. There must be a default entry
3) Look at your firewall with -iptables -L-. It should be wide open until you have solved your problem... (every chain should have accept status)
4) have you tried to look if the hostname resolution works? Try -dig www.yahoo.com-
or -host www.yahoo.com-
5) if not then look at your /etc/resolv.conf. There should be the entries of your provider DNS servers in it (you can see them when you look at /var/log/messages during connecting with ppp/pppoe.
Hope that helps.