What is the most standard subnet length on internet

Patrick W. Gilmore patrick at ianai.net
Fri Dec 19 06:07:08 CST 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 <ddunkin at netos.net>:
>>
>> 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.
>>
>





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