Mapping IP -> ASN

Lyndon Nerenberg wrote :
-> Is there a tool available that will take an IP address and return the
-> ASN attached to that network? I've been in situations before where this
-> information would be useful, but not having access to the BGP routing
-> information I'm not sure how to do this? (I've looked around some
-> "likely" web sites - such as Merit - but haven't found anything.)
-> The pressing need is for the ASN associated with This
-> network is connected to I*star, but I can't tell which of I*star's
-> three ASNs it's homed on. (And they aren't being responsive to queries.)

% whois -h

descr: ESYS-NET
origin: AS542
advisory: AS690 1:3561(11) 2:3561(144) 3:3561(27) 4:3561(218)
mnt-by: CANET-RC
changed: 960501
source: CANET

-> --lyndon