Iljitsch van Beijnum iljitsch at muada.com
Fri Jun 1 23:05:27 UTC 2007

On 2-jun-2007, at 0:43, Joel Jaeggli wrote:

> One of the potential values of unique private address space is the
> ability to built your own internets. Now whether there is value to
> unique but private address space that is significantly higher than
> private but non-unique address space (1918 style) or simply obtaining
> your own address space the normal way is a good question.  
> presumably an
> administrative hurdle has to be crossed in the former and later cases
> but not the middle one.

I think not everyone has a full understanding of why the IETF came up  
with unique local addressing for IPv6. The idea was NOT to create a  
new class of address space in addition to RFC 1918-style private  
addresses and regular globally routable address space. The main issue  
was that the existing equivalent of RFC 1918 in IPv6, site local  
addresses, required extensive special case handling in routers and  
applications, without a clear definition of how this was supposed to  
work in practice. See http://www.ietf.org/rfc/rfc3879.txt for the  

Other address types also require special case handling in IPv6 such  
as link local addresses. Every IPv6 system (host or router) is  
required to have an address in the prefix fe80::/64 on all of its  
interfaces. This means that the fe80::/64 prefix is present on more  
than one interface, which defies all previously known rules about  
routing. But since packets using those addresses aren't allowed to  
pass through a router, that's not really a problem.

The idea behind site local is the same, except that you can have a  
few router hops within a site. There is no convenient location where  
you can kill all site local packets so they don't leave the "site"  
like you can with link locals.

Additionally, there's the issue of organizations that each use local  
addressing and end up merging their networks. Non-unique addressing  
makes this very hard.

Solution: new type of local addresses that doesn't require any router  
magic to keep the packets within the site, and is globally unique so  
network merging isn't an issue.

This means that despite some different properties, ULA space is  
really the IPv6 equivalent of RFC 1918 space and NOT some kind or  
bastard invention that is secretly trying to be global space.

More information about the NANOG mailing list