You can install an application like nmap, located athttp://insecure.org/nmap/
. This can be installed on the server if you wish and run from the command line. Probably will give you more than you need but what the heck.
Not sure what your network consists of but if you have any Windoze boxes on there you can also use similar apps installed on one; something like AngryIP will do the trick.
In most cases you simply need to supply the IP range you want to scan and you'll get a report of connected hosts.