Nmap
From Steak Wiki
Nmap is the defacto standard port / network scanner. It is good.
There is a book available for nmap. I think there's a mostly complete pdf available online.
Ping Lan Address Range and Report who is Active
$ nmap -sP 192.168.1.0/24
Get DHCP Servers in LAN
$ sudo nmap --script broadcast-dhcp-discover -e eth0
Basic searches
The second thing to remember with nmap and searches. -T is tcp. -U is udp. -P is a ping (check that host is alive) test. So remembering these three simple switches, gives you the following commands
$ nmap -sP 192.168.1.0/24 $ nmap -sT -p1-1000 192.168.1.0/24 $ nmap -sU -p1-1000 192.168.1.0/24
port / service detection (searches first 1000 ports, can take minute or two) $ nmap -sV ipaddress