How do network drivers work?

[Chip Salzenberg]

I've often wondered about network drivers.  Normal device drivers just
provide low-level access to devices; but network drivers actually provide
file-level services.  How can they do this?

I'm especially interested in how this works with a system like SCO Xenix.
Xenix is distributed binary-only, but unlinked.  This allows you to add
drivers.  But how can an add-on driver splice itself into filename
resolution?  In other words, how do network drivers work?

[For curiosity's sake, if you know how NFS or RFS work, I'm interested in
that as well.]

PLEASE EMAIL TO ME.  I will summarize to the net.
-- 
Chip Salzenberg                 UUCP: "{codas,uunet}!ateng!chip"
A T Engineering                 My employer's opinions are a trade secret.
       "Anything that works is better than anything that doesn't."