IPv4 vs IPv6

On February 3rd, 2011, the Internet Assigned Numbers Authority (IANA) handed out the last blocks of IPv4 addresses to the five regional registries. After thirty years, the pool was empty. The internet's original address system had officially run dry.

And yet here we are, well into the 2020s, and most of the internet still runs on IPv4. New devices come online every second. Billions of IoT sensors, phones, and servers keep getting addresses. How?

Through a combination of NAT, CIDR, private address spaces, and dynamic allocation, all clever hacks that squeeze more life out of a fundamentally limited system. We've been patching the roof instead of building a new house.

IPv6 is that new house. It was designed in 1998, has been "coming soon" for over two decades, and offers enough addresses to assign a unique one to every atom on the surface of the Earth, several times over. The transition has been painfully slow, but it is genuinely happening now. Understanding both protocols and how they coexist is non-negotiable for anyone working in networking or cloud infrastructure.

IPv4 addresses look like this: 192.168.1.1, four groups of numbers (0-255), separated by dots. Each address is 32 bits long, giving us 2^32 = approximately 4.3 billion possible addresses.

IPv6 addresses look like this: 2001:0db8:85a3:0000:0000:8a2e:0370:7334, eight groups of four hexadecimal characters, separated by colons. Each address is 128 bits long, giving us 2^128 = approximately 340 undecillion (340 trillion trillion trillion) addresses.

To put that number in perspective: if you could assign a billion IPv6 addresses per second, it would take 10 billion trillion years to use them all. The sun will burn out long before we run out.

The fixed header size in IPv6 is a big deal for performance. IPv4 headers have variable lengths, which means routers have to parse each header to know where the data starts. IPv6 headers are always exactly 40 bytes, routers process them faster because there's no guesswork.