IP and port numbers

[Wen, Shixing]

Could someone explain how a machine can have more than one ip address? 

Shixing Wen

> ----------
> From: 	Nick Arnett
> Reply To: 	arnett at alink.net
> Sent: 	Thursday, May 28, 1998 9:01 AM
> To: 	Multiple recipients of list
> Subject: 	Re: IP and port numbers
> 
> 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
> --
> 
> Phone/fax: (408) 733-7613  E-mail: narnett at mccmedia.com
> 
> "Defy Demographics!"
>