[ale] FTP through a firewall?

[Joe Bayes]

Speaking of IP masq and such, is it possible for me to run a PPP line
from my linux box to an ISP, and then have another box connected to my
linux box, and be able to get access (telnet, ftp, whatever) to the
second box (the one not connected directly to the PPP link), from the
internet? 

If so, how would I do so? Would I have to buy an additional IP
address? How would I refer to the second box?

Just curious,

--joe