nested prefixes in Internet

Owen DeLong owen at
Tue Oct 11 22:58:09 UTC 2016

> On Oct 10, 2016, at 14:59 , Baldur Norddahl <baldur.norddahl at> wrote:
> Den 10/10/2016 kl. 22.27 skrev Owen DeLong:
>> Not true… There are myriad reasons that the /24 might not reach a network peered with ISP-A, including the possibility of being a downstream customer of a network peered with or buying transit from ISP-A. In the latter case, not an issue, since it’s paid transit, but in the former (peered, not transit), again, ISP-A is probably not super excited to carry traffic that someone isn’t paying them to carry.
> But ISP-A is in fact being paid to carry the traffic. Supposedly ISP-B has a paid transit relation to ISP-A. In the case the transit link is down ISP-A might have to transport the traffic through a less profitable link however.

Which isn’t really in the agreement between ISP-B and ISP-A unless it was specifically (and unusually) negotiated.

Also, you’re assuming that the leased space came with a transit agreement. In many cases, address leases don’t, so consider the additional scenario where ISP-B leases addresses from ISP-A, but has transit contracts with ISP-C and ISP-D but no connection at all to ISP-A.

> I know that if ISP-A was my network I would be making money even with the transit link down. Yes I might have to transport something out of my network through one of my transits, but outbound traffic is in fact free for us because we are heavy inbound loaded.

Yes, but it doesn’t help if it also came in on a transit link. Any traffic you both receive and transmit on transit costs you money pretty much no matter who you are.


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