[arin-ppml] CIDR v2.0

[Iljitsch van Beijnum]

In the early 1990s IPv4 address space was running out. They fixed this  
by changing routing protocols. Maybe we should try to do that again.

<history>

In the early days of the internet you could only use IPv4 addresses as  
blocks of 16777216, 65536 or 256 addresses. 256 was too small for most  
people so 65536 was a popular choice, but there are only some 16000 of  
these class B blocks and it was looking like those would be exhausted  
by the mid-1990s. So they started giving people a bunch of class C  
blocks (256 addresses each) but now the routing tables started to  
explode because a university that needed a single class B block before  
now used something like 16 class C blocks, which had to appear in  
routing tables individually. To fix this, routing protocols,  
especially BGP, were changed to be able to work with address blocks of  
arbitrary power of two sizes so address space and routing table slots  
could be managed much more efficiently. (This is "classless  
interdomain routing".)

</history>

Now that IPv4 address space is becoming scarce again, why not do the  
same thing again? But now rather than arbitrary powers of 2, we modify  
the protocols to work with arbitrary address ranges. I.e., 192.0.2.27  
- 192.0.2.43.

To accommodate legacy routers that can't do lookups based on ranges we  
can put in backward compatibility mechanisms from BGP5 to BGP4 similar  
to the ones from BGP4 (with CIDR) to BGP3 (no CIDR).