Of special note, Nmap 5.00 is now able to map IP addresses to AS numbers: "The script works by sending DNS TXT queries to a DNS server which in turn queries a third-party service provided by Team Cymru (team-cymru.org) using an in-addr.arpa style zone set up especially for use by Nmap.

The responses to these queries contain both Origin and Peer ASNs and their descriptions, displayed along with the BGP Prefix and Country Code."

http://nmap.org/nsedoc/scripts/asn-query.html