nmap

scan for IPs and ports

nmap -PR -oN /tmp/nmap.out 192.168.1.0/24

search hosts

nmap -sP 192.168.1.*
nmap -PR -oN /tmp/nmap.out 192.168.1.0/24

scan for open ports

nmap -T4 -sS 192.168.1.100

get more detailed information

sudo nmap -O 192.168.1.100 -T4

scan one port

nmap -sV -P0 -p 22 -vv 192.168.1.100

Links
http://linuxwiki.de/nmap