IP and port numbers

[Nick Arnett]

At 10:53 PM 5/27/98 -0700, 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.

IP addresses refer to a particular machine (one machine sometimes has more
than one IP address, but you probably don't have that issue). Port numbers
refer to logical connections on a machine, so the combination of an IP
address plus a port is always required to make a logical connection.  Lower
port numbers are considered "well-known" addresses because they are the
defaults for various services such as POP and SMTP for e-mail or HTTP for
the Web. For example, the default address for Web servers is 80. If you're
connecting to most Web servers, you don't have to include the "80" in the
address because the software does it for you as the default. You may have
noticed a few Web sites where there's a number after the address, separated
by a colon, such as "web.foo.com:8090". That means port 8090 instead of the
default of 80. The name "web.foo.com" is resolved by DNS to the IP address.

Your firewall probably blocks all but the most common well-known ports. It
may even block some well-known ones, such as POP, to prevent people from
trying to hack your system via e-mail clients. Since z39.50 is not widely
used, your firewall is probably blocking port 210, preventing sysetms
outside the firewall from connecting inside via that port.  If everything
was working prior to installing the firewall, chances are you just need to
change the firewall configuration to stop filtering port 210.  That should
be documented; it is the fundamental purpose of the firewall to allow you
to control these things.

Hope that helps...

Nick
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