[zAy0LfpBZLC8mAC]

> But why are there "lots of ways" to do it? Why isn't there one single, "it just works" way to add compatibility?

Well, for the same reason why there are lots of ways to tunnel IPv4 over IPv4? Some work through NAT, some don't, some encrypt, some don't, some allow for transport of packets that exceed the tunnel's transport PMTU, some don't, ... it's some form of tradeoff for different needs: If you are behind CGN of some crappy provider, you probably have to tunnel over UDP using small packets, if you are an ISP, that would be way too much overhead and you want to instead transport over raw IP in jumbo frames.

> Why isn't the IPv6 my ISP provides simply an extension of my IPv4? If my IPv4 is 216.58.213.174, why isn't the IPv6 216.58.213.174.[more stuff]? Why can I just concatenate my IPv4 public and NAT'ed LAN IP to get a valid IPv6?

Because that would not actually help anything. That would possibly make the user interface look a bit more familiar, but you would still need a completely new protocol underneath with all the same migration problems that we have now. And also, the differences at the user interface level are pretty minor, conceptually IPv6 really works much the same as IPv4, except with longer adresses. That they chose to use hex notation instead of decimal really is probably simply to make the notation shorter, and also the shorthand-notation for a run of zeros makes things a bit less unwieldy.

As for actually somehow reusing the existing address space, the existing global IPv4 routing table is massively fragmented with every larger ISP having tons of routes for disparate prefixes that they acquired over the years. That makes IPv4 backbone routers expensive. IPv6 is large enough that providers generally should never have more than one prefix, which makes the routing table a lot smaller.

[simias]

But don't you think UI is extremely important in these situations? If you sold IPv6 as "it works like IPv4 but with 128 bit addresses" I'm sure it wouldn't seem nearly as daunting to make the switch. But when you start looking closely you realize that it's more than that.

Regarding the size of the routing table, is it really that much of a big deal? Is it significant in the ISP's expanses? I find that hard to believe.

Also, obviously the ISPs might want to use clever tricks to save on bandwidth and processing power but that's not really what I'm talking about. ISPs have the manpower and the knowledge necessary to deal with that difficulty.

The case I have in mind is what is, in my experience, 99% of the private networks today: you have a single public address, a NAT/Firewall/Router combo and then a handful of private subnets.

This is where the migration should be as simple, seamless, familiar and comfortable as possible. This is where people can't be bothered to spend one week understanding IPv6 and reconfigure everything and still not be certain that they won't break something. This is where having IP addresses that actually look like IP addresses matter.

[zAy0LfpBZLC8mAC]

> But don't you think UI is extremely important in these situations?

Well, yeah, that's why they invented a more compact notation than 207.170.223.70.213.170.133.129.131.136.235.244.33.54.210.133.

> If you sold IPv6 as "it works like IPv4 but with 128 bit addresses" I'm sure it wouldn't seem nearly as daunting to make the switch.

But ... it doesn't seem daunting? I mean, obviously, you seem to think so, but I didn't find anything daunting about it when I enabled IPv6 on my network and servers ~ 15 years ago.

> Regarding the size of the routing table, is it really that much of a big deal? Is it significant in the ISP's expanses? I find that hard to believe.

The current global IPv4 routing table has ~ 670,000 entries, the current global IPv6 routing table has ~ 41,000 entries, distributed over 58,000 and 14,000 ASes, respectively.

Mind you that a router has to do a routing table lookup for every packet, so tens to hundreds of millions of lookups per second for backbone routers. The CAM for routing table storage is one of the performance limiting factors of a router, and the larger it needs to be, the slower and/or more expensive it gets.

> The case I have in mind is what is, in my experience, 99% of the private networks today: you have a single public address, a NAT/Firewall/Router combo and then a handful of private subnets.

There were 6to4 or teredo, for example, which allowed for some of that. But part of the problem is that if you want to talk to IPv6 hosts over v4-only connectivity, you need to talk to some sort of gateway--the IPv6 host itself obviously doesn't have an IPv4 address (otherwise you could just as well speak IPv4), you yourself cannot directly speak IPv6, so ... you need some party that is connected to the IPv4 and the IPv6 networks to translate between the two. If that is supposed to provide good performance, your ISP needs to operate such a gateway. Which means it's not exactly an automatic solution either.

> This is where the migration should be as simple, seamless, familiar and comfortable as possible. This is where people can't be bothered to spend one week understanding IPv6 and reconfigure everything and still not be certain that they won't break something.

Well, for the most part, most end users really shouldn't need to do anything like that. Even less so that with IPv4, really: Your ISP assigns you a prefix, your router assigns addresses to your devices, and everything should just work.

If you actually want to do some fancy network setup or something ... well, really, it's not that difficult. Easier than v4, really.

> This is where having IP addresses that actually look like IP addresses matter.

But they look exactly like what IP addresses have looked like for the last almost 20 years! IPv6 IP addresses, that is ...