Can multiple applications use the same port?

#The basic rule

At any given moment, one port plus one protocol (TCP or UDP) can be occupied by only one application.

This lets the operating system:

• unambiguously know
• which process to pass incoming data to

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#What this looks like in practice

#TCP (the most common case)

• One server:
  • listens on one port (for example, 80 or 443)
• Many clients:
  • connect to that port
  • each client uses its own temporary (ephemeral) port

Important:

• the server is one application
• the port is one
• the connections are many

This is not multiple applications, but one application with many connections.

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#Why two different applications cannot simply share a port

If:

• two different applications
• listened on the same TCP port

the OS:

• could not determine
• which application to hand the incoming segment to

So the OS forbids this by default.

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#Exceptions and nuances

#1. UDP + special settings

In UDP it is possible:

• to allow multiple applications
• to listen on one port (through special socket options)

But:

• this is a rare and specific case
• used in special scenarios (for example, multicast)

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#2. Different protocols

TCP and UDP are different port spaces.

So:

• one application can listen on TCP 53
• another on UDP 53

And this is normal.

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#3. Different IP addresses

If a server has:

• multiple IP addresses

then:

• different applications can listen
  • on the same port
  • but on different IPs

Example:

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#The key idea

The OS distinguishes connections by:

IP + port + protocol

And for TCP connections, by the full pair:

(client IP, client port) ↔ (server IP, server port)