Network Intersubjectives

> Both seem decentralized and distributed

Decentralized is being used in the sense that there is no single node that at the 'center' of the network (that is, a single node that all other nodes must connect to to participate in the network).

So, yes, a distributed network is 'merely' an extreme example of decentralization, but the term isn't usually used that way.

In practice, however, most successful distributed networks aren't 'pure', and have portions that look more like the decentralized diagram, at least temporarily. Sometimes this is forced by the underlying transport topology (eg. network 'backbones'), and sometimes this is an adaptive feature for increasing efficiency and throughput (eg. 'supernodes', etc.). In general, though, as long as such optimizations are adaptive and ephemeral, we'd still call such networks distributed.

Meanwhile, using email as a prototypical example of a decentralized network is somewhat misleading: if you only look at the servers, the network is actually distributed (any server can connect to any other). The lumpiness of decentralization only emerges when you add all the email clients. Except that these days most of the clients are web based and so the decentralization fades back into a distributed-ish model. The browser that you use to open your webmail account doesn't really count, as it isn't itself participating in the network of email protocols (POP, IMAP, SMTP).