Sure, sooner or later two networks will happen to generate the same prefix. When that happens -- and assuming those networks want to talk to each other, one of them simply generates a new prefix and renumbers. This is a significantly better situation than with RFC1918 (or SLAs) where a collision is _guaranteed_.
unmanaged delegations _will_ create collisions. and the problem is not when these sites want to talk w/ each other, its when your packets go to (one) of the other places using the identical prefix.
and then there is the nasty delusion of "Internet"... protestations to the contrary, the VSNL view of the "Internet" is vastly different than the US DOD view of the "Internet", is vastly different than the GE view, is different than the AS 701 view, is different than the Chinese R&E Network (CERN) view.... which one(s) count? Policy routing dictates that there is no such thing as a "global" routing table...
There are clearly many parts of the Internet that are "private" and one large part in the middle that is clearly "public". ULAs are intended to only be used within the "private" parts or even totally disconnected IP networks.
that model -might- have been accurate once, but has not been an accurate representation for several years. there is no middle,
For me, as long as I have IP reachability to those folks whom I want or need to talk to, I could care less about the "rest" of the folks using IP to move datagrams about ...
Exactly. However, the scope of who you want/need to talk to dictates what sort of addresses you need (with the current routing architecture) and where you get them.
the "scope" of who I want to talk to varies over time. just because the list of folks I want to talk to does not intersect w/ yours does not give you the right to tell me that I must use "private" or ULA or site-local addresses. we should each be able to be delegated address space which has -zero- chance of collison w/o a means to arbitrate. ULAs have no defined arbitration technique defined, other than through the legal system. RIR managed space has the arbitration technique as an intergral component of the delegation process. roughly - ULA == the lawless west RIR == civilized society -IF- ula space is ever approved, my advice to all transit providers is to never filter it.
S
Stephen Sprunk "Stupid people surround themselves with smart CCIE #3723 people. Smart people surround themselves with K5SSS smart people who disagree with them." --Aaron Sorkin