How to find all of an ISP's ASNs

Curtis Doty Curtis at
Thu Oct 27 21:38:30 UTC 2016

On Tue, Oct 25, 2016 at 9:03 PM, Hank Nussbacher <hank at>

> and if that doesn't work try:
> [replace the ASN with the ASN of your choice to see the interconnections.]

​Doesn't always work--as it will only show upstream ASNs.

For example, Comcast's backbone AS interconnects their regional ASNs.
However, the regionals don't show up on so
you'd need to find all of them first...with something like[search]=Comcast and/or consult your
favorite route server.

Also Gary, keep in mind these aren't static. I.e. new AS are added/removed
over time. And inferred policy (i.e. hub/spoke) could change too.

But I'm still to you propose to filter by AS?

And what if your neighbor (inside one of those permitted AS) is
compromised? You've just re-exposed your IoT devices' soft white underbelly
again. :-(


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