TCP and UDP services generally have a client-server relationship. For example, a TELNET server process initially sits idle at a system, waiting for an incoming connection. A user then interacts with a TELNET client process, which initiates a connection with the TELNET server. The client writes to the server, the server reads from the client and sends back its response. The client reads the response and reports back to the user. Thus, the connection is bidirectional and can be used for reading and writing.
How are multiple TELNET connections between two systems identified and coordinated? A TCP or UDP connection is uniquely identified by the following four items present in each message:
The port is a software construct that is used by the client or server for sending or receiving messages; a port is identified by a 16-bit number. Server processes are usually associated with a fixed port, e.g., 25 for SMTP or 6000 for X Windows; the port number is ``well-known'' because it, along with the destination IP address, needs to be used when initiating a connection to a particular host and service. Client processes, on the other hand, request a port number from the operating system when they begin execution; the port number is random although in some cases it is the next available port number.
As an example of how ports are used for sending and receiving messages,
consider the TELNET protocol.
The TELNET server listens for incoming messages on port 23, and sends
outgoing messages to port 23.
A TELNET client, on the same or different system, would first request
an unused port number from the operating system, and then use this port when
sending and receiving messages.
It would place this port number, say 3097, in packets destined for the
TELNET server so that the server, when responding to the client,
could place the client's port number in its TCP packets.
The client's host, upon receiving a message, would examine the port and
know which TELNET client should receive the message.
This is shown conceptually in figure 
.
Figure: TELNET Port, IP Interaction.
There is a somewhat-uniform rule that only privileged server processes, i.e., those processes that operate with UNIX superuser privileges, can use port numbers less than 1024 (referred to as privileged ports). Servers mostly use ports numbered less than 1024, whereas clients generally must request unprivileged port numbers from the operating system. Although this rule is not firm and is not required in the TCP/IP protocol specifications, BSD-based systems adhere to it. As an accidental but fortuitous result, firewalls can block or filter access to services by examining the port numbers in TCP or UDP packets and then routing or dropping the packet based on a policy that specifies which services are permitted or denied (this is covered in more detail in Chapter 2).
Not all TCP and UDP servers and clients use ports in as straightforward a fashion as TELNET, but in general the procedure described here is useful in the firewalls context. For example, many personal computer operating systems have no UNIX superuser concept, but still use ports as described (although there is no standard that requires this).