[ilkkao]

You can definitely craft an IP packet by hand and send it. If it's IPv4, you need to put a number between 0 and 255 to the protocol field from this list: https://www.iana.org/assignments/protocol-numbers/protocol-n...

Core routers don't inspect that field, NAT/ISP boxes can. I believe that with two suitable dedicated linux servers it is very possible to send and receive single custom IP packet between them even using 253 or 254 (= Use for experimentation and testing [RFC3692]) as the protocol number

[Twisol]

> 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".

[inglor_cz]

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.

[morcus]

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

[hylaride]

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

[Ekaros]

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.

[marcosdumay]

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?

[rfmoz]

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

[stavros]

Very interesting, thanks!