This is intended to be an ongoing list of use cases that come up rather than a proper post, so I won’t date updates to it. My use cases come largely from running servers and experimenting with devices at home.
(1) Running multiple servers at different domains on the same subnet on ports 80 and 443. These could even be different servers on the same machine: useful, for example, if you run a web server and a proxy server on the same ports, or maybe a PHP and a Java server platform – but you need more than one address, which is where IPv6 comes in. You can’t port forward different domains to different IP addresses using IPv4 and NAT unless you have multiple public IPv4 addresses and thus multiple network interfaces. As IPv4 addresses have run out, only organisations that bought large numbers ages ago have these. The cost of IPv4 addresses is high because of scarcity and small users can’t really hope to have more than one or two if they are lucky and can pay. There are literally trillions of IPv6 addresses available per person.
(2) Publicly addressing multiple devices in a household. Use cases could be IP security cameras, just your mobile and tablet, or any Internet-enabled device. This isn’t a security risk if you have a properly configured IPv6 firewall in addition to your IPv4 one, which your router will already be doing for you. The addresses are rather hard to guess if you have trillions of them, and IPv6 devices are usually allowed by the router to choose their own addresses, unlike IPv4 address leases from the router, so they change anyway. Adds up to better security.
(3) As some countries find it impossible to get IPv4 addresses, they will switch to IPv6-only networks. You cannot access the IPv6 network from IPv4 and vice versa. All servers and home networks can run IPv6 but, as IPv4 addresses cannot be obtained, increasingly they will not all be able to use IPv4 everywhere in the world, so those still running it will be invisible to them. The current stop-gap fix of running dual-stack networks is eventually going to break because it is limited by needing an IPv4 address for each server or network, which means it is mathematically no more effective than simply running IPv4-only networks as previously.
(4) The proliferation of devices per person running on multiple networks (home broadband, work broadband or VPN, phone data connection) will simply make IPv4 and other stop-gap solutions like carrier-grade NAT unworkable in the long run.
(5) The solution is well established and could have been put in place long ago: there is no technical reason not to have done it as routers have already been replaced and software can be updated even on old devices to allow IPv6 networking.
(6) Although for many use cases IPv6 simply replaces IPv6 as an addressing system and does not impact on speed, in others it will speed up the Internet because it is more efficient for certain things like addressing multiple devices on a subnet. The technologies that could be developed using IPv6 are not being developed because so many people can’t use them while IPv4 is still being used in parallel with it.
(7) The ordinary user will only see a better Internet but will not have to do anything to use IPv6 and retire IPv4. Software and systems will be updated so it will happen for them. In most cases this has already happened. We only need to plan now to switch off IPv4. Otherwise, sooner or later, it will happen anyway. A third of Internet traffic is IPv6. Soon the small amount of IPV6-only traffic (i.e. which cannot be on IPv4 because it is to locations without IPv4 addresses) will in time grow, just as general IPv6 traffic has done over the last few years. As soon as this includes major services that millions use, IPv4’s days are done. If Google or Facebook abandons IPv4, so will the rest of the world except for legacy purposes, largely confined to internal subnets. Having a public IPv4 address, once many millions of people cannot see it, will be virtually pointless. It will not be especially useful for hiding Internet traffic since security agencies will still be able to monitor what is left of the IPv4 network; it is easiest to find people in a place with almost no traffic. Perhaps IPv4 will be switched off – it doesn’t really matter when that happens, as soon as everybody has moved to IPv6 anyway.