docker network — connect containers to each other

Quick Answer

# Create a network
docker network create my-net

# Run containers on the same network
docker run -d --name db --network my-net postgres:16
docker run -d --name app --network my-net -p 3000:3000 my-app

# Containers can now reach each other by name
# app → postgres://db:5432/mydb

Usage

Containers on the default bridge network cannot resolve each other by name. User-defined networks add automatic DNS.

Other causes & fixes

Inspect a network

docker network ls
docker network inspect my-net

Connect a running container to a network

docker network connect my-net existing-container

Docker Compose — automatic networking

Compose creates a default network for each project. Services can reach each other by service name.

# docker-compose.yml
services:
  db:
    image: postgres:16
  app:
    image: my-app
    environment:
      DATABASE_URL: postgres://db:5432/mydb
# No explicit network needed — Compose handles it

Expose a port to the host

# -p HOST_PORT:CONTAINER_PORT
docker run -p 8080:3000 my-app
# Access at http://localhost:8080