Why didn’t they just start out with IPv6?
24 bits is a lot of addresses, even though whole blocks are reserved for specific uses.
PCs were not around at the time, and private ownership of computers was also in the future.
“Nobody will ever need more than 640k RAM!” - Bill Gates, 1981
Because back then computers were big, expensive machines and IPv4 was considered very generous.
Back when v4 was created, a 32-bit address space was a BIG freakin’ deal. It was considered flippin’ HUGE.
At the time, the IMP (Internet Message Processors) were 16-bit minicomputers with less CPU power than your cellphone. Mainframes were the only computers with 32-bit address spaces or even native 32-bit math operations. The prevailing 16-bit minicomputers used for routing processors needed to handle the 32-bit addresses (remember, there is one for destination and one for source) in four 16-bit wide chunks.
Going to 128 bit addresses back then would have made everything significantly more slow than it was. Again, remember there is a source and destination address, so we’re talking 256 bits (or 16 16-bit words) of fetching, never mind processing, just to deal with the address block.
Minicomputers back then typically had no more than 64KB of memory, and usually 8kB of that was reserved for IO address space, so you really had only 56kB of usable memory.
No one wasted even a byte back then. No one could have foreseen the explosion of what was ARPAnet into a worldwide network. ARPAnet wasn’t even available to the civilian market in the US, and you had to be apply or be invited to join it in the 70’s.
Most network address schemes back then didn’t have even 32 bits. XNS had only 24 bits of address space, and it was hacked up into an eight bit network address and 16 bits for the station. DECnet was similar, but even smaller, AppleTalk was like XNS, etc. X.25 addresses were overlaid on telephone numbers, because telco switches were used to route the circuit calls.
As someone who started doing computer networks in ‘84, I can tell you that none of us thought it would get this big. No one. None of us thought there might be a day when the utilities were going to try to assign an IP address to ever meter on their system, no one thought of using IP packet networks to carry phone calls, no one thought of IP as being in your TV VCR equivalent.
Not even the most starry-eyed dreamers back then envisioned this explosion of the IP network - or ANY network, for that matter.