If you want to scan a network or range of IPs for hosts that are on a network, you can use nmap with the following option
nmap -sL NETWORK/MASK (ex: nmap -sL 192.168.1.0/24)
or
nmap -sL NETWORKRANGE (ex: nmap -sL 192.168.1.0-50)
For a complete reference of nmap capabilities, go to http://insecure.org/nmap/man/.