What is IPv4?

Overview

IPv4, Internet Protocol version 4, is the fourth revision of the Internet Protocol and the first to be deployed at scale. It was defined in 1981 by RFC 791, and more than forty years later it still carries the majority of traffic on the internet.

Its design has aged remarkably well. Its address size has not.

The address format

An IPv4 address is a 32-bit number. Writing it as 32 binary digits would be unreadable, so it is split into four 8-bit groups called octets, each converted to decimal and joined with dots. This is called dotted-decimal notation.

192.168.1.1      a private address, probably your router
203.0.113.42     reserved for documentation, which is why we use it in examples
8.8.8.8          a public DNS resolver run by Google
127.0.0.1        loopback, meaning this machine

Thirty-two bits gives 2³², or 4,294,967,296 addresses. Just over 4.3 billion. In 1981, when the entire network was a few hundred research and military hosts, that was an unimaginable surplus.

Classful addressing, and why it was abandoned

The original scheme carved the address space into fixed classes. The first bits of an address decided how much of it identified the network and how much identified the host on it.

The problem is visible the moment you look at the gaps. An organization with 300 machines was too big for a Class C and got a Class B instead, wasting roughly 64,700 addresses. There was nothing in between. The scheme burned through the address space at a rate that had nothing to do with actual demand.

CIDR, Classless Inter-Domain Routing, replaced it in 1993. Instead of three fixed sizes, a block could be any power of two, written with a suffix saying how many bits identify the network. A /24 is 256 addresses, a /23 is 512, a /22 is 1,024. That organization with 300 machines gets a /23 and wastes about 200 addresses instead of 64,700.

Running out

CIDR bought years. It did not buy enough. As the internet moved from research network to household utility, and then to a thing inside every phone, thermostat, and doorbell, 4.3 billion stopped being a large number.

Two techniques carried the load in the meantime, and both are still doing so today.

NAT

Network Address Translation lets an entire household or office sit behind a single public address. Your router rewrites the return address on the way out and reverses it on the way back. It is the reason your provider can serve a home with fifteen connected devices using exactly one address. See Public IP vs Private IP for how that works in practice.

Reclamation and transfer

Blocks allocated generously in the 1980s have been returned, subdivided, and sold. There is a real market in IPv4 space, and prices have climbed steadily. An address that was free to request in 1990 is now an asset on a balance sheet.

Reserved ranges worth recognizing

Some IPv4 ranges never appear as a public address. Knowing them on sight saves a lot of confused debugging.