Many of our readers are no doubt familiar with the basics of IP address: They provide a network address that's used to reliably route Internet traffic to your PC, smartphone, or other device. The problem, though, is that the Internet is almost out of available IP addresses.
IPv4 is the current main IP (Internet Protocol) technology. Anyone who connects to the Internet gets assigned an IP address, which is up to 12 digits long, IPv4 technology allows for roughly 4 billion individual IP addresses. And not all IP addresses are created equal: There are "classes" of IP address--some intended for public use (usually to identify servers on the Web), and others for private use (like devices connected to your home network). For example, the IP address for Google.com as compared to a typical Netgear router (like you might have in your home).
Google's public IP address appears to be 126.96.36.199 at the time of writing, and Netgear routers, by default, can be reached from inside a network at 192.168.1.1. The "192." IP block, known as "Class C", is designated for private use, usually intended to be used with some type of NAT ("Network Address Translation"), to allow many devices to operate behind a single network access point (think of multiple PCs connecting to the Internet via one Wi-Fi network).
Class C addresses are reusable, since they exist only within a private network. The Google.com IP address, on the other hand, is "Class A" since it begins with "66.". Class A addresses are generally not reusable, eventually leading to complete depletion of the available address space as more IP addresses are requested and used.
Unfortunately, that time is now! At the rate new IP addresses are being claimed, the common IPv4 public address space--the pool of total available IPv4 addresses--could be completely depleted within weeks.
No, this doesn't mean the Internet is over.
Some clever folks predicted this problem with IPv4, and developed IPv6 (don't ask what happened to v5) to have an exponentially larger address space (that is, since it can support longer IP addresses, it can allow for more IP addresses--a lot more). This page gives a graphical comparison of just how many more IP addresses IPv6 supports. Google has already flipped the switch on many of their services, and some ISPs are already silently rolling out IPv6 to subscribers. And most recent operating systems provide at least basic IPv6 support. The Internet Society has a more in-depth FAQ about IPv6, and how you can enable it on your PC.
Hopefully, the transition will go so smoothly that no one will notice... until they find their IP address now contains letters! That's right, not only will IP addresses be longer, but your IP address will now be in hexadecimal. You did pay attention in math class, right?