monitor lan traffic

Can somone tell what I need to have installed on suse linux machine so that I can maybe monitor an analize some of the traffic on my network. I'm wanting to see if I can determine what is causing so much traffic on the lan at times that it makes things crawl. I know there is some software on the dvd/cd to do this just not sure what it is. If know what it is I do not mind going an reading the man pages to see if i can get it going or a good how-to on the subject either. A good recommendation on a good firewall book / howto would be nice to have also. thanks for any help you list member can give me. jack malone Network Administrator EAST TEXAS LIGHTHOUSE FOR THE BLIND dba HORIZON INDUSTRIES 903-595-3444 http://www.horizonind.com