My take is that NAT's respond to several flaws in the IPv4 architecture:
- 1) Not enough addresses - this being the one that brought them into
existence.
- 1a) Local allocation of addresses - a variant of the preceeding one, but
subtly different; NAT's do allow you to allocate more addresses
locally without going back to a central number allocation authority,
which is very convenient.
- 2) Easy renumbering when switching ISP's - a benefit that only was
realized later in time, but a significant one all the same -
especially for those people who reckon that switching addresses is a
really painful undertaking.
You might be surprised to find that I agree with the above. But I also think
it's useful to consider technical reasons for wanting NATs in IPv4 separately
from the things that motivate people to install NATs - and many of the latter
have no technical basis. e.g. people have been sold on NAT functionality by
being told that it's a firewall and that the NAT protects them from attacks.
I think that if you look at the points I listed above, the market has
clearly decided that IPv4+NAT (for all its problems, with which people are
I'm sure reasonably familiar, given the many years NAT has been in service
widely) is the most cost-effective solution to providing them.
Again, there are sound technical justifications and there are uninformed
motiviations, and both affect people's purchasing decisions. Whether NAT+IPv4
is cost-effective has a lot to do with what you want to do with your network;
the answer will be different for different user communities. I think it's
more realistic to say that IPv4+NAT was the easiest thing to try first to
alleviate the flaws you list above, since it claimed (unrealistically) to
solve those problems without upgrading hosts, applications, or the packet
format.