Finding ASN from IP address

[Avleen Vig <lists-nanog () silverwraith com>]

I want to create a mapping of IP addresses to ASN, for a specific like
of IP addresses. Eg:
  1.2.3.4
  12.34.56.78

etc, gathered from my system logs.

What is the best way of doing this?

I thought about something along the lines of:
  install routing software (zebra?)
  pass software the IP's, get it to spit back a string from which I can
    grab the ASN

Two problems being I don't know which software to install that can do
that, or where to get a copy of the current routing table, so that I can
feed that to the software.

Suggestions appreciated.

-- 
Avleen Vig
Systems Administrator
Personal: www.silverwraith.com
EFnet:    irc.mindspring.com