We've received a /48 allocation from ARIN, and want to advertise smaller blocks from different sites in our organization. I tried advertising a /56 from each site, but ISP is telling me they won't advertise anything smaller than a /48. So each site would have to have its own /48.

Has anyone else run into a similar issue with their ISP?

  • 2
    By the way, even though ARIN issued a /48, they did reserve, at least, the /44 (possibly the /40) for you for growth. It should be easy to get a larger network.
    – Ron Maupin
    Sep 23, 2015 at 18:22

1 Answer 1


Yes. Nobody will advertise any network smaller than a /48. The same thing is in IPv4; no ISP will advertise any network smaller than a /24.

The problem is the sheer number of routes that would generate on the Internet by advertising networks smaller than a /48. Even at /48 with the current global address range of 2000::/3, that translates to 35,184,372,088,832 possible routes.

In fact, IANA had a rule that nobody (even individuals) should get networks smaller than /48. The ISPs eventually overruled that for individuals, giving them /56. Each site is supposed to have at least a /56, and a company gets at least a /48 which the company can advertise. If you wish to advertise individual sites, they each need a /48 (or a /56 out of separate /48s).

You can go to the RIR and get a smaller mask. For instance, we got a /32 from each RIR.

  • In real live people do advertise strange things. I'm currently seeing 18 /56 and 7 /64 on a test router. For your own protection you should filter for those.
    – user2084
    Oct 20, 2018 at 8:21
  • @JensLink, I wonder if those are just advertised within the ISP AS. It is unlikely that another ISP will accept such between ASes. There are also IPv6 martian/bogon lists that should be filtered.
    – Ron Maupin
    Oct 20, 2018 at 15:54
  • I have no idea where these are coming from. It's just a test Linux box where I don't run any filtering. But I should talk to the upstream provider
    – user2084
    Oct 20, 2018 at 16:38

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