[arin-ppml] ARIN-prop-127: Shared Transition Space for IPv4 Address Extension

Matthew Kaufman matthew at matthew.at
Thu Jan 20 18:44:55 EST 2011

On 1/20/2011 11:42 AM, William Herrin wrote:
> Setting aside one
> space that every ISP can use seems like a smarter solution than each
> ISP carving out a piece of their otherwise publicly routable space for
> the task.


And I'll note that if *any* ISP carves out a piece of their space for 
this, *any other* ISP can reuse exactly the same space.

In fact, a bunch of ISPs could get together and simply apply for space 
now, or out of the transition-reserved pool, or buy it through the 
transfer market for this purpose.

> I just programmed, 172.16,0.0/12 and on my
> network. My ISPs are now conclusively shown to have a conflict with my
> active and valid internal addressing.
> Seriously Wes, that's not a sound argument. There are only 70,000
> /24's in RFC1918 space and folks who don't take their home routers'
> default often select much more than a /24. Of _course_ any ISP of
> non-trivial size is going to conflict with some of his customers if
> they're both selecting from the same relatively small pool.

As far as I can tell, if you don't care about your customers always 
being able to talk to each other the collision space is only against a 
single /30, not a whole /24, of RFC1918 space, if you're doing it right.

And if your customers do want to talk to each other, you can make that 
work with much less than a /24 worth of collision space as well, though 
some customers who make certain internal address assignment and routing 
decisions will be unable to reach others.

> This is a red herring. Customers who deliberately break their networks
> are in a whole different category than the ones where I've
> inadvertently stomped on their reasonable and legitimate
> configuration.

Well, once my ISP starts assigning out of, say,, it might not 
be "reasonable and legitimate" for my customers to expect that using indiscriminately internally any more.

Transition is going to be painful and require changes. This is, in some 
cases, one of those changes.

Matthew Kaufman

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