I've never taken a CS class, and I still know it. I've just stared at enough packet dumps and debugged enough issues that it happens to stick.
I think it's a stupid question to ask as a screener, as it tests familiarity with trivia. It's the kind of thing I'd be happy to see that somebody knew, but it's never something I'd downcheck them for not knowing. It could just mean, as in your case, that there deep knowledge is somewhere else.
We are all a product of our environment. And there is certainly a chance in 10 years the OP will still know how many bytes are in a MAC address. I only mentioned computer science based on the "student" part of the quote.
Personally I've been spoiled by working on higher level stuff so if I do look at a packet dump I usually filter the packet headers out. But again, we're a product of our environment and asking questions that a reasonable skilled engineer might go their entire career without knowing the answer to except in that interview is a bit suspect and will likely disqualify people that would be otherwise great for the job.
Maybe 10 years from now I'll have forgotten everything about the hex/binary/octal representations of numbers, but I certainly hope not!
Uh actually my classes didn't teach me that. Side projects did.
The point still stands though. First, it is much easier to have side projects when you are in school so the causal relationship might still be there just one step removed. But the more important point I was making was that school or a side project or even a project at work, if people go 10 years without using it, they forget things.
As an aside, you may want to drop the "Uh" in-front of your sentences. It conveys a certain tone that I'm not sure you are doing intentionally.
That is why you know the answer. Come back in 10 years and let us know if you still know it. What you think might be mainstream in a computer science class are rarely used in application. And if they are they can be easily looked up.
I used to know the exact effective distance of a CATV cable when I was a student. Useful? Sure. Something I need to remember for the rest of my life? Definitely not.
As another example, in 17+ years writing ISO level 7 programs, I have never once needed to use the Mac address.