Are there different ports for output and input?

Question

When sending data using UDP, a destination port is needed to be specified.

If sending by TCP, a source port should also be specified.

Are there different ports for input and output? E.g., if I specify port 1234, can I use it for both input and output or should I use different ports for output and input?

EDIT: To clarify my question: - I send data from port X. - Someone sends data to me to port X. Are those two different ports or is the same one used?

Answer 1

The source port is a port that exists only on the computer that is initiating the connection, whereas the destination port exists only on the computer that is receiving it (though both are visible to both endpoints). Both TCP and UDP have both source and destination ports. Usually the source port is selected automatically by the socket library from the unused ports on the computer. There are very few good reasons for selecting a specific source port, and it will often be changed by the Internet gateway (router) as a part of the Network Address Translation (NAT) process.

Edit: To clarify, both the source and destination ports are used for both input and output. Which port is on your computer depends on which end of the TCP connection you are on. If you are on the receiving end, then the destination port is on your computer. When you are looking at the connection from your perspective, it will be the source port, and will be used for both input and output. The same principle applies to UDP as well, except that there are no "connections" per se, merely an exchange of raw data between ports.

Answer 2

TCP needs both a source and a destination port because it forms a connection between the two clients, whereas UDP is connectionless; You simply send data to a destination port and it either arrives or not.

So with TCP, you open a "channel" between two computers. You send data through it and possibly receive some back.

With UDP, if you want to receive data, then yes you need a "separate" port that listens for incoming data.

Answer 3

When sending data using UDP, a destination port is needed to be specified.

Correct.

If sending by TCP, a source port should also be specified.

Incorrect. The system will allocate one for you automatically if not specified. This is the normal usage.

Are there different ports for input and output?

No. The local port you are bound to is used for both.

And all this applies to both UDP and TCP.