Because "just add 4 more bytes of address space" wouldn't be backwards-compatible with the world's worth of hardware and software built around IPv4 anyways. The

Because "just add 4 more bytes of address space" wouldn't be backwards-compatible with the world's worth of hardware and software built around IPv4 anyways. There just isn't anywhere in an ipv4 packet that you could put those extra bytes that wouldn't break existing systems. So the only real option is creating a new protocol with a new protocol ID in the header. And if we're making a new protocol anyways, we might as well design it to fix more problems than just address space exhaustion. And that's what IPv6 is. The Wikipedia article has a lot of details on the kinds of changes it makes and why: https://en.wikipedia.org/wiki/IPv6#Comparison_with_IPv4 https://en.wikipedia.org/wiki/IPv6#Comparison_with_IPv4