Mapping IP -> ASN

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.)


is always useful. So is



You can look at Internet routing table collected by Merit.
(""). But the table
may mislead you to a wrong ASN if the IP address is an inet-rtr. My
suggestion is consulting the table first, and if you think that one IP
might be a inet-rtr, consulting IRR using "whois -h