nmap -sS hostname
scans open ports on hostname

nmap -sT hostname
scans open ports on hostname. Faster, but 'noisier'

nmap -sU hostname
scans open UDP ports on hostname

iptables -L -v
shows your iptables rules. The -v verbose option shows the interfaces and stats.

arp [hostip]
spits back the MAC address for that IP

Someday try...

• arpwatch - shows low level network traffic to resolve IPs and MACs

-- MattWalsh - 29 Nov 2004