Hi,
you are right - the laptop will most definitly have a place where you can put your ethernet cord - but that doesnt mean it´s really active.
Inside the laptop is a interchangeable card with the functions I mentioned above - that one determines wheter you have the capability to operate modem, Lan or both.
I sold my A21 ages ago but I hope I remember the right place for that card.
When you look at the bottom of your A21 there are some covers which you can remove - it´s only 1 or 2 screws each so you can´t make anything wrong - open the covers - remove them (either ground yourself before touching anything inside or try to avoid touching anything inside (not a problem) because static electricity could damage your laptop).
You´ll see probably your RAM (small, lengthy cards - longer side with some pins where they connect to the mainbord) - the one you are looking for looks the same - but is only half the length and is at least as high as long (hope I am not too confusing).
Write down what you can read (maybe the information is placed on a folded piece of plastic - you can unfold it - don´t worry about static e. - it´s plastic) - and drop me a line or look for yourself on the internet - then you can determine what card you have and what´s possible.
Cheers, Engin