Networking and Monitoring software (OT)

I am curious if there is any software out that can monitor a network of
computers and tell you the ip numbers of each computer, and who
is actively using the network?

After the Hacker/Cracker  and Firewall discussions, I wanted to ask this,
because originally when our computers were networked.  A "FRIEND"
helped us get our stuff set up.  Shortly after this and prior to our
Firewall.
A person, unknown to us, accessed one of our systems and disabled our
access to this machine.  It took a complete Fdisk and reload to gain control
over that computer again.

Now we have three computers on our network that are numbered in sequence,
however once in a while this other number appears.  I personally would like
to know where this other number is coming from.  Actually as of recent there
has
been a second rogue number to appear.  I want to know if this "friend"
sort of inserted himself into our network somehow someway.  Does a
monitoring
software exist, that can tell us how many users are on OUR little network
and all
that good info?

Sincerely,

Nancy Whittley

HWG hwg-techniques mailing list archives, maintained by Webmasters @ IWA