i suspect the OP is down the rabbit hole of what is known as "anti-aliasing," trying to find out whether IP address A on some router is actually on the same router as IP address B, and what AS(s) those IPs are in. your point is that an inter-as link may have IPs from either of the providers. yup. and, because it is an INTER-as link, it does not really belong to one or t'other. this particular rabbit digs deep holes. an early entrance to the burrow is the classic from the uw crew inproceedings{Spring:2002:MIT:633025.633039, author = {Spring, Neil and Mahajan, Ratul and Wetherall, David}, title = {Measuring ISP Topologies with Rocketfuel}, booktitle = {Proceedings of the 2002 Conference on Applications, Technologies, Architectures, and Protocols for Computer Communications}, series = {SIGCOMM '02}, year = {2002}, isbn = {1-58113-570-X}, location = {Pittsburgh, Pennsylvania, USA}, pages = {133--145}, numpages = {13}, url = {http://doi.acm.org/10.1145/633025.633039}, doi = {10.1145/633025.633039}, acmid = {633039}, publisher = {ACM}, address = {New York, NY, USA}, } randy