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Port Numbers

When doing any kind of networking operation such as setting up or connecting to a server, the subject *Port Numbers arises.

Here’s what you should know about port numbers for working with web servers (or other network servers):

  • Transport layer protocols are the rules that govern how two processes (running programs) on machines connected to a network communicate with one another.

  • The internet, as we have known it from the 1980s to today in the early 2020s, has been based primarily on two transport layer protocols: TCP and UDP. Both of these protocols use Port Numbers in their communication.

  • For each network interface, there are up to 65535 channels numbered 1-65535, and these channels are called Ports or Port Numbers.

  • The upper bound 65535 is a hard limit, because the space for the port number in the TCP and UDP packet headers is a 16 bit field.

  • Port 0 does not actually exist (it’s an invalid port number) and port 1-1023 are typically reserved for privileged processes. (meaning that you need system administrator privileges to use them.). It’s generally a good idea to stay away from 1-1023 for casual testing and development.

  • The default port for a web server is TCP port 80, which is a privileged port. When setting up a test webserver, it’s typical to use port 8080. However, sometimes other ports are used, including 3000, 4000, 5000, 8000, or 8081. You can use any port number as long as it isn’t being used for another purpose.

Error Port already in use

On any given host, only one process can use a port at a time. So if you already have a webserver running on 8080 in one window, and you try to start up a second one, you’ll get an error message such as Port already in use.


  • Shut down the other process before starting a new one using the same port
  • Use a different port

Ports and Spring Boot

When running a Spring Boot server, we typically define the port to be used in the file src/main/resources/ with the following line of code:


This line of code signifies that the server.port property should be defined as the value of the environment variable PORT if it exists; otherwise it should be the value 8080

Coding it without the environment variable fallback, as server.port=8080 will work on localhost, but will fail on Heroku. On Heroku, the cloud server will assign web servers (whether Spring Boot, Python Flask, Rails, or whatever) a port that they are expected to run on by specifying the PORT environment variable. If the server does not start up on the expected port within some reasonable timeout, say 15-20 seconds, then the process will be killed an error will be logged.

Therefore when running Spring Boot on Heroku, the server.port=${PORT:8080} form should always be used.