The ports you keep looking up

Every developer eventually asks the same questions: which port does PostgreSQL use, is 9092 Kafka or something else, what is the debugging port for Java, why does the app refuse to start because 8080 is taken. This sheet collects the ports of the services you meet in Java, container and DevOps work, with the protocol and a short note, and a search box to filter them.

The Kind column matters: well-known (below 1024) and registered (1024 and up) mean the port is listed by IANA for that service, while default means that the product commonly ships with it but it is not an official registration, so it can be changed freely and may clash with something else. Spring Boot's 8080 is both; JDWP's 5005 is only a convention.

Working with ports

• Only one process can listen on a given address and port. "Address already in use" means another process, or a previous run still shutting down, has it.
• A service bound to 127.0.0.1 is reachable only from the same machine; bind to 0.0.0.0 to accept outside connections (and then protect it with a firewall).
• In containers, the port inside the container and the published host port are independent: -p 8081:8080 publishes container port 8080 on host port 8081.

The table is a reference, not a security policy: check your vendor's documentation for the version you run. It is static and works offline.

Frequently asked questions

What is the difference between well-known and registered ports?

The IANA registry divides ports into well-known (0 to 1023, historically needing root), registered (1024 to 49151) and dynamic or private (49152 to 65535). A "default" port in this table is the usual value a product ships with, which may not be registered.

Why does 8080 appear everywhere?

Ports below 1024 historically needed administrator rights, so application servers such as Tomcat chose 8080 as an easy alternative to 80. Spring Boot kept that default.

Which ports should never be open to the internet?

Databases (3306, 5432, 1433, 1521, 27017), caches (6379, 11211), remote desktop (3389), SMB (445), the Kubernetes kubelet and etcd. Put them behind a private network, VPN or bastion host.

How do I see what is listening on a port?

On Linux use ss -ltnp or lsof -i :8080; on Windows use netstat -ano | findstr :8080; on macOS use lsof -iTCP -sTCP:LISTEN.

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