It is probably an option that can be set in whatever driver you are trying to use.

A Google search for <"virtual pc" "dc21x4" ip dos> suggests that it is in protocol.ini. (Yes, that's probably a Windows thing usually, but apparently it works for DOS too.)
http://www.winvistatips.com/dos-networking-vpc-t521121.html

.ini is just a text data file with CR separated values, has nothing to do with being a 'windows thing' or any OS, except for the fact that linux text files are CR+LF (or vice versa, never can remember which is which).

It's a bit more than "[newline] separated values", it's a primitive database of key+key+value sets.

[section]
key=value
;comment

I think Jorpho was referring to protocol.ini itself being more likely to be found in Windows, rather than in DOS.
The MS TCP/IP network drivers for DOS do have a protocol.ini
So if that's what the OP is using that's where he needs to look.