When an interviewer asks what is potentially a very broad question, then (as other answers mention) it should really be about having an ongoing conversation to find out how much detail they want, as well as making sensible inferences about what technical level the interviewer is at and what kind of detail might be most relevant to your role.
For example, an embedded developer might be expected to know the algorithms for TCP/IP handshaking, whereas I'd guess a network engineer is probably expected to talk more about routing. A web developer meanwhile might be expected to talk more about higher-level protocols, such as HTTP requests and responses. I'm none of these things, so I'm only guessing what goes for common sense topics in these fields.
Either way, start off with a short sketch that touches on several aspects, and (if you're capable of going into considerably more detail) then just ask them if they want you to add more detail, and just rinse and repeat until told to stop (or until you can announce that either the explanation is complete, or that you're at the limit of your own understanding).