tcp        0      0 0.0.0.0:631             0.0.0.0:*               LISTEN      5831/cupsd          
udp        0      0 0.0.0.0:631             0.0.0.0:*                           5831/cupsd         


nmap -p 631 127.0.0.1

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2006-12-21 18:33 CET
Interesting ports on INFINITY.ZONE (127.0.0.1):
PORT    STATE SERVICE
631/tcp open  ipp

Nmap finished: 1 IP address (1 host up) scanned in 0.247 seconds