Node, Java, or curl reporting connect ECONNREFUSED 127.0.0.1:3306? It means nothing is listening on that port. Covers service not running, wrong port, 127.0.0.1-only binding, Docker container networking, and firewall REJECT rules — with ss/lsof diagnostic commands.

ECONNREFUSED: Connection Refused — 5 Causes Explained (Including Docker)

docker run fails with port is already allocated or bind: address already in use? The host-side port you mapped is taken — by another container's docker-proxy or a host process. How to tell the two apart, the compose project-name collision scenario, and why 127.0.0.1 and a specific IP binding can coexist.

Docker port is already allocated: Finding and Freeing the Taken Port

What does CIDR like 192.168.1.0/24 mean, how to get the subnet mask, how to compute usable hosts? Explains subnetting with intuitive examples, plus our CIDR / subnet calculator.

#networking

ECONNREFUSED: Connection Refused — 5 Causes Explained (Including Docker)

Docker port is already allocated: Finding and Freeing the Taken Port

Subnetting / CIDR Explained: What Is 192.168.1.0/24 and How to Calculate