IP and port numbers

[mbancroft at ssc.nsw.gov.au]

We are working to make our catalogue available on the web via a Z39.50
gateway (or z-client). We are running a z-daemon on our library server.
I am searching for a good tutorial on how IP addresses work in conjunction
with port numbers.
The problem is that since a firewall was installed on our corporate LAN we
cannot hit our database.
I am confused about addressing and port numbers. Where do the port numbers
(210 in the case of Z39.50) come into play?
The IP address of our library alphaserver is obviously different from the
IP address of the firewall which sits in front of it.
Is our target address, e.g.  the_ firewall:210 ?

TIA

Max Bancroft
Sutherland Shire Libraries and Information Service.
Sutherland, NSW Australia.