IPv6 in 60 seconds

Why IPv4 ran out

An IPv4 address is 32 bits. Each bit is a 0 or a 1, so there are 2³² possible addresses: 4,294,967,296, about 4.3 billion. That sounds like a lot until you compare it with roughly 8 billion people, and then add every phone, laptop, games console and smart fridge they own.

IPv4 addresses are handed out from the top down. IANA keeps the central pool and gives large blocks to five regional internet registries, which pass them on to internet providers and organizations. On 3 February 2011, IANA gave the last five blocks to the registries, and the central pool was empty. The registries then hit their own limits one by one in the years that followed.

IPv4 didn't stop working. Existing addresses keep working, and tricks like NAT (a later episode) let many devices share one public address. But the long-term fix is a bigger address.

128 bits

An IPv6 address is 128 bits long, four times as many bits as IPv4. Every extra bit doubles the number of addresses, so 2¹²⁸ is a number 39 digits long:

340,282,366,920,938,463,463,374,607,431,768,211,456

That's about 3.4 × 10³⁸ addresses. Enough to give every device on the planet its own address, with plenty to spare.

How to read an IPv6 address

Writing 128 bits in dotted decimal would be painful, so IPv6 uses hexadecimal: the digits 0 to 9 plus the letters A to F, where each hex digit stands for 4 bits. The address is split into eight groups of four hex digits, separated by colons:

2001:0db8:0000:0000:0000:0000:0000:0001

Each group is 16 bits, and 8 × 16 = 128. It looks long, but there are rules for shortening it, and you'll learn to read it quickly. (The 2001:db8:: range in this example is reserved for documentation, so it never appears on the real internet.)

Try it yourself

See whether your device has an IPv6 address:

• Windows: open Command Prompt and run ipconfig. Look for "IPv6 Address" or "Link-local IPv6 Address".
• Mac: System Settings → Network → your connection → Details → TCP/IP, and look under IPv6.
• Linux: run ip -6 addr.

Count the colons and groups. If a group seems to be missing, the address has been shortened; the next episodes show how.

Good to know

• The IANA central free pool ran out on 3 February 2011, when the last five /8 blocks went to the regional registries; the registries ran out at different times after that.
• NAT has stretched IPv4 for years, and IPv4 and IPv6 run side by side today.
• 2³² = 4,294,967,296; 2¹²⁸ ≈ 3.4 × 10³⁸, a 39-digit number.
• The example address uses the documentation prefix 2001:db8::/32.

For CCNA learners

This lines up with CCNA 200-301 exam topic 1.8, configure and verify IPv6 addressing and prefix: know that an IPv6 address is 128 bits, written as eight 16-bit groups of hex digits separated by colons.

Answer to the comment question

A full IPv6 address has 32 hex digits. It has 8 groups, and each group has 4 hex digits, so 8 × 4 = 32. Each hex digit stands for 4 bits, which is why 32 × 4 = 128 bits.