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 198.161.92.0. 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.) --lyndon