Iptables is a firewall, installed by default on all official Ubuntu distributions (Ubuntu, Kubuntu, Xubuntu). When you install Ubuntu, iptables is there, but it allows all traffic by default. Ubuntu 8.04 Comes with ufw - program for managing a netfilter firewall

There is a wealth of information available about iptables, but much of it is fairly complex, and if you want to do a few basic things, this How To is for you.

https://help.ubuntu.com/community/IptablesHowTo