Lets take this sample situation (which happens all the time).

Fledgling ISP A wants a block of address space, ISP A goes to the NIC directly.
The NIC says go to your upstream NSP B. NSP B is a regional and is ready to give
out a /19, but ISP A doesn't want to be tied down to a particular NSP.
Time goes by. ISP A goes to the NIC again and makes an
embelished case to the NIC (thinking that the NIC will give ISP A a /19). Instead
the NIC give ISP A a /20. ISP A doesn't have a clue about Sprint's filters (or
anyone elses filters). NSP B informs ISP A that the /20 is too long of a prefix
to pass through Sprint's filters. ISP A cires bloody murder, and appeals to the
NIC asking for the next /20 contiguous to the first one and the NIC says no way.
Finally ISP A gets a /19 from NSP B and HAS to renumber. At this point ISP A has
renumbered three times (first from a /24, then from a NIC assigned /20, and then
finially to a NSP assigned /19). ISP A is still clueless and doesn't even send in
the SWIP updates for the block from NSP B. ISP A exhausts the /19 due to inefficient
address space utilization and asks for another /19. NSP B says, I see no SWIPs for
your block, as far as I can tell you havn't even used it yet.

So why did ISP A want NIC assigned address space in the first place? Because, he
didn't want to have to renumber anymore after getting it. That way he could go
to any NSP and be routed and not have to re-number all his customers. So it all
boils down to not wanting to ever have to re-number. Maybe ISP A should talk to
the people at pier :-). But seriously, renumbering is more of a preceived problem
than it actually is. The ISP's customers would have to renumber if they switched
ISPs anyways, so if they are truly happy with the ISP, then they have no reason to
leave.

Lets face it, if you are starting out you may have to renumber several times if you
are trying to get NIC assigned address space. If you get provider assigned address
space you will have to renumber if you change providers.

The next problem is if you want to be multi-homed (through different NSPs) you will
want NIC assigned address space. So lets apply this to the case of the small ISP,

* a small ISP shouldn't be concerned about being multi-homed. There are
much bigger issues than wasting money and time to become multi-homed.

* when the ISP is ready to become multi-homed then it will be big enough
to justify large blocks and will have to renumber.

* Maybe the problem is reliabilty of the primary link, (say his NSP has
The ISP should find a reliable provider and renumber.

So, if you are a startup ISP and grow and want to become multi-homed you will renumber.

It all boils down to renumbering, its a fact of life for a startup ISP.

Enough about startup ISPs, what about the case of the corporation that wants/needs
to be multi-homed. The easiest way is to just get multiple connections to the same
national backbone. This may not be possible in all cases, so you have to get NIC assigned
address space, so you have to renumber.

Renumbering is a fact of life, so live with it and find ways to do it better and more
efficiently.

Scott