Quick! Run around, panic, place your head in the sand, we've ran out of IPv4 web addresses! It's the end of the internet as we know it. OK... not quite, truthfully it's more of a notable point in history, though could cause issues for some ISPs, particularly smaller firms with out-dated hardware and limited budgets.
For every device on the internet, be it a PC, tablet or smartphone, there's a unique address, known as an IPv4 (Internet Protocol version 4) address. When the IPv4 standard was established in 1981, the notion that there could be billions of computers mere decades on seemed somewhat far-fetched and thus it was decided, to implement an address scheme with a theoretical maximum of just over four billion addresses.
33 years on, however, and Europe has officially reached the end of its slice of those four billion addresses. Had ISPs not prepared for this, firms would have no choice but to stop offering internet services for their new products. ISPs are free to chose their own solutions to the address shortage issue, with many choosing to implement the new IPv6 standard, currently tunnelling existing IPv4 connections over the new protocol to maintain compatibility. Other ISPs are resorting to NAT (Network Address Translation), the same technology used to allow computers on a local network to access the internet under a single address.
Ultimately, full IPv6 migration will be the only way forward. It's the closest alternative to be supported in most modern hardware and offers the most efficient long-term solution. IPv6 solves our addressing problem by offering up a number of addresses so high, that, *fingers crossed* we should never come even remotely close to reaching the theoretical maximum, which is 340 undecillion unique values (that's 340,000,000,000,000,000,000,000,000,000,000,000,000 addresses).
IPv6 accomplishes this huge number by using 128-bit addresses, four times the size of current 32-bit addresses. Initially, this may seem like a huge inefficiency, an extra 12 bytes per data packet, just for the address, a typically unacceptable overhead. However, IPv6's huge address range makes some seriously cool stuff possible.
Currently, businesses and home computers sit behind a NAT router. This router performs address translation to a single internet-side IP address. This is a slow process and can lead to complex configurations for the routing of incoming data to various devices, introducing overheads of its own. With IPv6, however, the notion of NAT will eventually disappear; every computer will be able to have a unique, internet-facing IP address. Gone will be configuration and many of the popular fire-walling and port-forwarding woes. Likewise, other changes to the protocol make it less computationally intensive to route, which could lower the cost/power consumption of user-end modems/routers though, most importantly, enables far more cost-effective and efficient carrier-grade equipment, ideal for high-speed international fibre routing.
It may be a while yet before your household or business is switched to IPv6 - though the European IP address body, Ripe, has ran out of addresses, many ISPs still have addresses assigned to themselves that are unused. Somewhere down the line, however, expect a new router or a letter with new configuration details through the door, to switch you over to IPv6.