Migration and interoperability, IMHO, are very minor problems. Software needs very minor changes to support IPv6 addresses (the socket interfaces are not affected, only the address input string, which is handled by an OS library...). The problem is 1/3 hardware/software (a lot of firewalls and NATs) and 2/3 political: most ISPs began as telephone or cable providers with a vested interest in creating or maintaining media distribution monopolies. NAT is the best thing since sliced bread to them since it largely breaks end-to-end routing/global addressability. (VoIP, P2P, etc, are all much harder through NAT)
You miss my point they should have extended v4 and not tried to do a totally new standard.
As the register commented "IPv6 was neither designed for small biz nor consumers. IPv6 was designed by big-ticket network engineers bearing global infrastructure and enormous enterprise networks in mind. Learned gentlemen who live in a world where buying IBM and connecting it with Cisco never got anyone fired"
They have reinvented the OSI stack and we know how well that worked in practice ( I was third line for the UK's X.400 so I know what OSI is like)
There were some NAT proposals, but I don't think any made it into the standard.
There is just no reason to use NAT if you have enough addresses. It's a hack to solve address scarcity, and doesn't add any security or any other benefits (unless you don't have a firewall, but you've got much bigger problems in that case!).
The RFCs for IP allocation say that every end site should get a /56 allocation - that is 256 subnets of /64 addresses [1]. A business site should be able to get a /48 (65,536 /64 networks) for no extra cost. Perhaps a mobile device with a cellular modem would get a /64 but that is the smallest allocation.
Sure, but there's not really much reason too (IPv6 still has private/non-routable addresses, so you might want to). NAT on IPv4 is used somewhat like a firewall - because there's nothing to configure - whereas with IPv6 the address space is large enough that there's (almost) no reason to use NAT (that I find convincing), and if a firewall is still desired, that can be run independability (for example, ip6tables).
Edit: To respond to your other comment, no, there's nothing stopping ISPs from inflicting NAT on IPv6 too, other than the consumer asking "why am I behind NAT when there is no shortage of addresses?".
Sure, but if the ISPs number-one goal is putting a stranglehold on your personal freedoms, is there anything actually stopping them from using NAT on IPv6 if they wanted to?
If they are truly draconian cabals of evil, I don't expect "well, you don't need it as much" would stop them.
