On 12 Nov 99, at 9:45, Roy L. Jacobs wrote:
> I am new at this and could use some assistance. I have read there
> are some 65,535 ports in tcp/ip, but have been unable to find
> anything which explains this. For instance, port 139 is ascribed by
> the tcp/ip protocol to net bios, but beyond that, I have found no
> further explanation. Does this port refer to a memory address on
> the receiving computer, or is it just some sort of sub-address.
> Where I am lost is when one claims that a certain port is "open" on
> a particular computer, what does that mean? I would greatly
> appreciate some help so I can be pointed in the right direction
> for further study. Thank you.
TCP and UDP "ports" are not physical constructs; they're "virtual"
objects which allow multiple sessions/conversations to share the same
physical connection. Each packet that arrives using either of these
protocols (indicated by a "protocol" field in its headers...) is also
distinguished by a "port number" field, by which the protocol stack
on the receiving machine can determine which communicating
process/instance is intended to receive the packet. Most port
numbers are assigned more-or-less at random, with the exception that
when beginning a conversation/session, the recipient port number will
generally be one of the "well-known" or "registered" ports, selected
because the intended recipient functionality is expected to already
be bound to that port and awaiting such connections.
[Thus the frequent question: Someone is trying to connect to my
port X; what service do they expect to find there?]
David G
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