> If it's IPv4, you need to put a number between 0 and 255 to the protocol field from this list:

To save a skim (though it's an interesting list!), protocol codes 253 and 254 are suitable "for experimentation and testing".

This is an interesting list; it makes you appreciate just how many obscure protocols have died out in practice. Evolution in networks seems to mimic evolution in nature quite well.

What happens when the remaining 104 unassigned protocol numbers are exhausted?

We're about half-way to exhausted, but a huge chunk of the ones assigned are long deprecated and/or proprietary technologies and could conceivably be reassigned. Assignment now is obviously a lot more conservative than it was in the 1980s.

There is sometimes drama with it, though. Awhile back, the OpenBSD guys created CARP as a fully open source router failover protocol, but couldn't get an official IP number and ended up using the same one as VRRP. There's also a lot of historical animosity that some companies got numbers for proprietary protocols (eg Cisco got one for its then-proprietary EIGRP).

https://en.wikipedia.org/wiki/List_of_IP_protocol_numbers

Probably use of some type of options. Up to 320 bits, so I think there is reasonable amount of space there for good while. Ofc, this makes really messy processing, but with current hardware not impossible.

People will start overloading the numbers.

I do hope we'll have stopped using IPv4 by then... But well, a decade after address exhaustion we are still on it, so who knows?

IPv6 uses the exact same 8-bit codes as IPv4.

It uses them a little differently -- in IPv4, there is one protocol per packet, while in IPv6, "protocols" can be chained in a mechanism called extension headers -- but this actually makes the problem of number exhaustion more acute.

What if extension headers made it better? We could come up with a protocol consisting solely of a larger Next Header field and chain this pseudo header with the actual payload whenever the protocol number is > 255. The same idea could also be used in IPv4.

I didn't mean to imply otherwise. But, as you say, this is equally applicable to IPv4 and IPv6. There were a lot of issues solved by IPv6, but "have even more room for non-TCP/UDP transports" wasn't one of them (and didn't need to be, tbqh).

I agree, although I am a bit surprised to see that we’ve used more than half of the protocol numbers already

Playing with protocol number change usually results in “Protocol Unreachable” or “Malformed Packet” from your OS.

Very interesting, thanks!