BT has started trialling IPv6 in small regions of the UK, alongside its usual IPv4 network. This comes a few weeks after Sky started pushing out a firmware update to enable IPv6 on some customers' routers. There's still no timeline for a full-scale IPv6 switch-over in the UK (or indeed anywhere else in the world), but with last week's news that North America is officially out of new IPv4 addresses it will need to happen sooner rather than later.

Last week, some BT FTTC (fibre-to-the-cabinet) subscribers noticed that their modems had been assigned an IPv6 address, in addition to the usual IPv4 address. In a statement to ISP Review, a BT spokesperson confirmed that the IPv6 trial was ongoing "with a small group of BT employees, before moving on to trials with customers at a later stage." Along with those BT employees (yay, dogfooding!), some customers were also receiving an IPv6 address because BT had to "fully enable very small parts of the network for IPv6 for a limited period of time."

As you may know, there's a fairly small number of total IPv4 addresses available—about 4.3 billion in total, though many of those are reserved for various non-public uses. IPv4 addresses, which are managed by regional internet registries (RIRs), were originally handed out in a fairly willy-nilly fashion: if you wanted a block of addresses, it was pretty easy to get one.

In the early '90s, however, when the number of Internet users started to explode in earnest, address exhaustion started to become a topic of discussion. By the mid-2000s, that discussion took on a slightly more desperate tone as it became clear that the number of Internet users was still accelerating at a massive rate.

In more recent years, IPv4 address allocation has been a lot more frugal, but it's still only a matter of time until they fully dry up: ARIN (North America) has recently run out, and RIPE (Europe), LATNIC (Latin America), and APNIC (Asia) aren't far behind. It's hard to predict when those RIRs will actually be exhausted, but it's most likely a matter of months rather than years.

The solution to IPv4 exhaustion is simple: switching to IPv6, which uses 128-bit addressing for a total of 2128 addresses (about 340 undecillion—that is, 340 with 36 zeroes after it). In practice, it's easy to switch new devices to IPv6, but that doesn't mean that billions of older devices stop using IPv4... and all of those old devices still need to be able to talk to the new ones. As a result, the current path towards IPv6 is hybridisation, or "dual-stack" as it's often known: devices that are capable of using both IPv4 and IPv6—and ISPs, data centres, and backbone providers rolling out new networking gear that can bridge and translate between IPv4 and IPv6 networks.

Most modern devices support both IPv4 and IPv6, and the equipment suppliers are ready to go with their dual-stack solutions. Now we're simply waiting for the ISPs and other Web service providers to: a) reconfigure and test everything out; and b) stump up lots of cash to buy and retrofit dual-stack equipment.

This small-scale test by BT is a sign that it's working on its IPv6 network, but don't get your hopes up for a full IPv6 deployment any time soon: "BT has a strategy to include IPv6 for all its customers in good time," quoth the spokesperson. It's the same story at Sky, which has been dabbling with IPv6 trials for a while, but has no official timeline for mass rollout.

If you want to use IPv6 in the UK, there are a handful of smaller ISPs that will provide an IPv6 pipe from your house to the Internet, such Andrews & Arnold. Some large Web service providers are IPv6-enabled, such as Google, but the vast majority are not.