IPv4, today's Internet workhorse protocol, offers more than 4 billion IP addresses. This may seem like a lot, but in a world of 7 billion people and every single toaster seemingly wanting an Internet connection, it's actually not. In fact, IANA, the Internet's numbering authority, allocated the last block of IPv4 addresses about six years ago. That doesn't mean there are no spare IPv4 addresses left on the planet – regional operators still have some reserves – but the supply already has been exhausted.

IPv6 comes to the rescue. With 128-bit addresses – that is, about a quadrillion IPs per every human body cell in the world, we'll hopefully be on the safe side for some time. However, IPv6 offers much more than extra bits in the address; it fixes a 35-year irritant of IPv4 operation by allowing the network to function without network address translation (NAT) or DHCP. Moreover, it enjoys being a first-class citizen in your Linux box.

Back to Basics

In general, IPv6 addresses are shown as eight 2-byte, colon-delimited hexadecimal numbers (e.g., fe80:0000:0000:0000:eef4:bbff:fe29:873d). So many numbers are awkward, so a few simplifications are possible. First, you can omit leading zeros: 0001 is the same as 1, and 0000 is the same as 0. Second, you can drop running sequences of all zeros, which are quite common in IPv6 addresses. You can do this only once per address for the longest sequence, though. A double colon shows where zeros were: consider fe80::eef4:bbff:fe29:873d or even ::1, which is just a loop-back address, serving the same purposes as 127.0.0.1 in IPv4. This means a single IPv6 address may have several equally valid representations. If you think IPv6 addresses are more difficult to parse than IPv4, that's true. Luckily, libraries such as glibc handle this for you.

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