I would say "no cost".
On the hosting service I use, you can rent an IPv6 only host and you pay less. Why would you pay for an IPv4 address if you don't need it ?
On the hosting service I use, you can rent an IPv6 only host and you pay less. Why would you pay for an IPv4 address if you don't need it ?
Last I recall (years ago) my buddy was running some type of VPN through Comcast just to be able to use IPv6 properly. That's when I decided it was still too new for me.
Clearly I don't know anything on the subject, and I'm not claiming to. These are one of the areas where I don't know, don't desire to know, and want it to "just work". I am however interested in making my applications compliant asap, but until I can reliably use it, I've not even attempted.
So with that big pile of ignorance, are we "there yet"? Ie, could I build applications and run them on a IPv6 compatible host, with hit it with my home ISP with a reasonable expectation that everything will work? (assuming my code works, of course).
I feel like I need a "caniuseIPv6.com" site, similar to https://caniuse.com. From the outside, IPv6 implementation and support seems bizarrely cryptic.
On the modem side, nothing needed to be done for IPv6. Any firmware updates or configuration needed is done by Comcast.
On the router side, there is a dedicated IPv6 configuration page. Settings are:
• Connection Type: Native
• All other available settings: Set to either 'Enable' or 'Stateless'.
That was it. My Mac has IPv6 configuration set to configure 'Automatically', and I get an IP. If I go to plain Google and ask it "what's my ip" I get a IPv6 IP back.
On my iPhone, on wifi, if I do the same "what's my ip" Google search, I get an IPv6 back. My carrier is Ting GSM; if I turn off wifi and do the same Google search, I get a different IPv6 IP back.
At work, IPv6 is rolling out _very_ slowly, but I was able to get a fixed IPv6 address. That has been programmed into the configuration for my laptop's USB-Ethernet adapter, so I have IPv6 at my desk at work. Although most work services are not IPv6-enabled, DNS is.
And at home, IPv6 is enabled and is in active use, both on desktop and on mobile.
As for testing, I suggest https://test-ipv6.com and https://ipv6test.google.com
(Yes, all IPv4 addresses can be represented as IPv6 addresses in a reserved zero prefix network, some network APIs just offer IPv6 and then treat IPv4 addresses this way for convenience, but we obviously don't route packets this way since those would be IPv6 packets, yet they're for an IPv4 destination which can't read them)
In some very large deployments they do v6-only. Everything internally is IPv6, when you connect to that IPv4 only site you'd actually connect to a company-provided gateway that speaks IPv6 on your side but IPv4 to the outside world.
If you're big enough this makes loads of sense - now you have this vast address space for everything, all your systems are simply configured for IPv6 only (not twice the configuration) and it pretty much just works. You buy some specialist appliances at the edge for that translation, but your users only have IPv6 stuff.
I'd welcome someone showing me how that was basically impossible, but I don't see why the entire IPv4 space isn't in a special prefix / address space of ipv6 since they allegedly have so many atoms of mappings.
Neither of these changes are required if there is a dual stack proxy or CDN that sits infront of your application, as they will most likely talk to your application over IPv4.
The other gotcha is if you try to interrogate clientIP for analytics, authorization, geo-ip etc, which may need a little more care.
(As a comcast customer that "one day" for me was several years ago)
Lots of things fail if you're IPv6 only, but it was a useful experiment and I'll try it again in a year or two.
Right now most of my hosts are dual-stack, and I have a /64 setup for my home network.