Any one know of any good free network monitoring software?

most of stuff i checked on google seems to be crap,trials or adware, hard to find a good one, I have 5 ip addresses on my network id like to have one program that can monitor stuff, im currently using k9 web protection but that has to be installed on each ,its not very handy, two of the ips are consoles and 3 computers, im basically looking for a security type program that can keep an eye on bandwidth and intruders stuff like that, if you know of anything like that please let me know.