What is the most standard subnet length on internet

[jabley at hopcount.ca (Joe Abley)]

On 2008-12-19, at 00:27, 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

I think in practice that's over-stating the problem.

If an RIR assigns you a /22, the chances are good it has been assigned  
from some larger block which is also used to assign longer prefixes,  
down to whatever the RIR's minimum is (e.g. /24 under common critical  
infrastructure policies).

While it's possible to imagine someone re-parsing a full set of all  
RIR data every day and rolling out martian filters to all border  
routers based on precisely what assignments have been made, that  
someone would incur that operational cost in the face of what is a  
fairly slim benefit seems unlikely.

More likely that someone would filter based on the longest assignment  
made in a particular /8 (e.g. in 202/7, 199/8 we might expect to see / 
24s, in 76/8 not so much, etc).

Even more likely than that is that people might filter out obvious  
RFC3330-style martians and permit everything else up to a /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.

We're both speculating, of course.

It'd be nice if some grad student somewhere with friends in the  
operations community was to experiment with /24s carved out of larger  
blocks from all over the planet and present some empirical data.


Joe