Why two HTTP and TCP addresses can use the same port and two IPC addresses cannot use the same named pipe?

What I think of a port is: Whenever a message arrives to a machine, it is copied to a memory area which is mapped to the port specified and the concerned application or service is notified that a message has arrived for it.

If this is true, then what happ开发者_开发问答ens if two messages arrive for two different services listening on the same port ? ( either http or tcp )

And why can not two named pipe addresses use the same named pipe ?

TCP identifies "connections" via a tuple of { local ip, local port, remote ip, remote port }. Therefore, since each incoming connection has a different remote ip/port pair, your local machine can distinguish between them.

HTTP uses TCP for its transport. Thus, an HTTP port is a TCP port.

If you've ever had your machine get a new IP address while you had connections open, you'll note that they break the first time they send any data out since the remote host does not recognize the (new) address and sends a RST response.

A pipe has only its name to distinguish it so there is only one "connection" no matter how many writers it has.

Your description is one way to handle incoming messages.

In the case of two web sites listening on the same port, there is one web server listening on that port, which then looks at the http host header to find the correct web site to forward the request to.

The same is true for named pipes, the RPC listener listens on the TCP port, and then finds out that it is a named pipe message and then forwards the message to the right named pipe.