IP and port numbers

[Chuck Bearden]

On Thu, 28 May 1998, Bob Cherry wrote:

> At 10:36 AM 5/28/98 -0700, Wen, Shixing wrote:
> >Could someone explain how a machine can have more than one ip address? 
> >
> >Shixing Wen
> 
> Many systems have more than one IP for a variety of reasons.  One generally
> requires two ethernet controllers if the system is to operate on two
> different networks however, a single IP can be used as long as both IPs are
> within the same network realm.
> 
> Port numbers (sockets) are totally independant from IP addresses.  Your
> "services" file defines which services use what ports.  

Close.  /etc/services maps a service name to a port number and
protocol.  It is quite possible to run a service without it being in
the /etc/services file, and it is quite possible to run a service on a
given port other than the one actually mapped to that port in 
/etc/services.

>                                                      In addition, access
> lists (used with router configurations) can determine which
> ethernet/internet addresses support which ports.  

Not exactly.  Router access lists can block traffic to and from hosts
based on IP address, transport layer protocol, port number, and other
criteria, but they have no effect on whether the host in question 
supports a given service on a given port.  The only way to prevent a 
computer from running a service on a certain port is to configure the 
computer itself not to.  AFAIK, router access lists don't operate on 
ethernet addresses, but am ready to stand corrected on that point.

>                                                   The Policy Editor may be
> able to give you some additional control (Windows95/NT) but I haven't
> entered that world yet.

The Win95 Policy Editor doesn't give you that kind of control,
unfortunately.  It will let you make Network Neighborhood disappear,
however.

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