[Eyas]

Right. The key innovation of the internet was the invention of TCP/IP for the purpose of inter-network communication. See https://en.wikipedia.org/wiki/Internet_protocol_suite#Early_...

[ethbro]

I'd say the kernel of that was the idea of encapsulation and separating functions by layer (really the same idea, from packet and flow perspectives, respectively).

[LoSboccacc]

I'd say the core is that anyone could link up and start talking with other parts of the network; the arp/ip technology was the stepping stone of course, but a key differentiator from previous machine networks[1] is that you can just hook up to the internet, it was truly decentralized from its ideation.

[1] ie. stock tickers, terminals of time share mainframes, or see plan 55-A for a wider interconnected switched network this russian idea