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Re: What is the most standard subnet length on internet

  • From: Patrick W. Gilmore
  • Date: Fri Dec 19 07:07:24 2008
On Dec 19, 2008, at 12:27 AM, Suresh Ramasubramanian wrote:

Even if a longer prefix like a /24 is announced, chances of people
accepting it is slim.   Especially, as you say, if the RIR allocation
is something larger than /24

And I have a feeling acceptance /24 route announcements of anything
other than legacy classful space, infrastructure space like the root
servers is going to be patchy at best.

If you are worried about /24s (and I really don't think you need to worry that much), just announce the covering CIDR somewhere and the few places that don't hear the /24 will send packets "at" the shorter prefix. Since routing is hop-based, as soon as the packet reaches an AS that hears the /24, the packet will be forwarded to the correct destination. I know from personal experience this works perfectly well today.

But in all seriousness, /24s are close to universally heard. Networks used to filter them, but by and large, they went away or changed their policy. Of course, there are hold-outs, but most corporations which own networks realize that the Internet is a tool to make money, not prove or disprove some random technical argument on NANOG. Listening to /24s make most networks money (either directly by giving them more traffic for which they charge their downstreams, or indirectly by having networks - like mine - stop using them if they don't), ... well, the rest is left as an exercise for the reader.

As for routing table size, no router which can handle 10s of Gbps is at all bothered by the size of the global table, so only edge devices or stub networks are in danger of needing to filter /24s. And both of those can (should?) have something called a "default route", making it completely irrelevant whether they hear the /24s anyway.

--
TTFN,
patrick


2008/12/19 Darryl Dunkin <[email protected]>:

If you are allocated a /22, announce the /22. Do not announce anything longer unless you have a requirement to (such as a different origin AS). If you are further allocating a subset of that to a downstream, then a /24 out of that is acceptable as the origin will be different.