ISPs like hathway,youtele,inet provide dynamic IPs and authentication is based on MAC address.You are also given username and password for connecting to internet.It is very easy to connect to PPP in ubuntu.
1)in terminal type:
and follow the steps in it.
2)You are done!
If you want to connect to internet than type:
sudo pon dsl-provider
If you want to disconnect from inter net than type:
sudo poff -a kill
(It will forcefully close active connection)