How to check the open ports in Linux

Two articles on the Internet are found:

http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/security-guide/s1-server-ports.html

http://www.cyberciti.biz/faq/how-do-i-find-out-what-ports-are-listeningopen-on-my-linuxfreebsd-server/