FW: /8s and filtering

william at elan.net william at elan.net
Tue Dec 10 21:09:13 UTC 2002

Well, you can't easily multihome when announcing /23 or shorter but /24 
will work fine for multihoming and that is why ARIN passed that policy.

What is true, however, is that some isps will filter even /24s on their 
router, but in that case, there would still be a route to your netblock 
from your upstream (they would be announcing their entire /19, /18 or 
whatever with you as a customer getting /24 of that larger block and if 
your direct route is filtered by an isp, next larger route that includes 
your block that is not filtered would be used). And there are actually not 
that many isps that really do filter /24 announcements (I'd say 1% but I 
maybe wrong). 

However that some ISPs do filter /24s, would mean that if RIR is directly 
giving your an ip block it would need to be from the block where ISPs know 
that RIR is giving out small blocks. Up until now ARIN has not been giving 
any new small (/24 - /21) allocations, except micro allocations for 
exchange points and in the micro-allocations (policy 2001-3) it is 
specifically written that ARIN will do these kind of allocations from 
specific blocks reserved for that purpose.

Now, however, that ARIN is discussing proposals such as 2002-5, 2002-6 and 
2002-7 (with 2002-5 & 2002-6 most likely being passed within few months)
ARIN maybe put in position of assigning smaller then /20 blocks and that 
is why I suggested on ARIN ppml mailing list that current micro-allocation 
wording about assiging small blocks from specifically designated larger 
blocks be made a separate policy that would apply to all small allocations
& asignments being made directly by ARIN. If you think its a good idea to 
make this a policy, please do send your feedback to ARIN or bring it up 
on ppml mailing list and then ARIN can work on this futher to make it a 

On Tue, 10 Dec 2002, Forrest wrote:

> > 
> > >I was also curious about this - if I am a customer who wants to 
> > >multihome and can justify only a /24, I would go to an ISP which has an 
> > >allocation from the Class C space rather than one from the Class A 
> > >space.
> > 
> > 	It doesn't matter. For all practical purposes, basement multihomers
> > only 
> > care that their two or three providers have their route.
> Maybe I'm missing something, but what good would it do for someone to 
> multihome if only their own providers accept their route, but nobody else 
> does?  I realize that their block should be still announced with their 
> ISP's larger aggregate, but what good does this do if your ISP goes down 
> and can't announce the large aggregate.  
> If you're a smaller organization, perhaps you'll only have a /23 from your 
> upstream provider.  With the filtering that seems to be in place, it seems 
> like the only way you can truly multihome with a /23 is if it happens to 
> be in the old Class C space.  Or is this wrong?  
> What seems to be needed is perhaps a /8 set aside by the RIR specifically 
> to allocate to small organizations that wish to multihome that people 
> would accept /24 and shorter from.  
> Forrest 

More information about the NANOG mailing list