IP and port numbers

[Chuck Bearden]

On Wed, 27 May 1998 mbancroft at ssc.nsw.gov.au wrote:

> 
> 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.

Some firewalls do packet filtering of one form or another.  Nick
Arnett addressed the situation regarding those firewalls.  

Other firewalls use network address translation (NAT) or IP
masquerading as part of their security.  Packets from machines behind
the firewall are rewritten either to use an IP address from one of a
pool of IP addresses assigned by the firewall (in the case of NAT) or
to use the IP address of the firewall itself (in the case of IP 
masquerading).  Is it possible that your institution is using one of
these forms of firewalling?  Could that be why you are asking if the
target address is firewall:210?

In either NAT or IP masquerading, machines behind the firewall aren't 
accessible from the outside under their actual IP addresses, and 
enabling inbound traffic is more complicated than with the various
forms of untranslated firewalling.  I think it is still doable in some
fashion, but whether that fashion satisfies those responsible for your
network security is another matter.

Chuck Bearden
Network Services Librarian
Houston Public Library
Houston, TX  77002
713/247-2264 (voice)
713/247-1182 (fax)
cbearden at hpl.lib.tx.us