Docker networking, without the guesswork

Written by the Novosys Academy team · Explainer

When a container starts, Docker attaches it to a network. Which network it joins decides what it can reach and how it gets found.

The three you will see first

• bridge: the default. Containers get private addresses on a virtual bridge. Outside traffic reaches them only through published ports.
• host: the container shares the host's network stack. No port mapping is needed, and no isolation either.
• none: no network at all. Useful for jobs that should not talk to anything.

Publishing a port

A container listens on its own port. To reach it from your machine, map a host port to it:

docker run -d -p 8080:80 --name web nginx
# host port 8080 -> container port 80

Finding other containers by name

The default bridge does not give containers DNS names. A user-defined network does. Put both containers on the same one and they find each other by name:

docker network create app-net
docker run -d --network app-net --name db postgres:16
docker run -d --network app-net --name api my-api:1.0
# inside api, the host name 'db' resolves to the database container

This is the single most useful networking habit in Docker: create a named network for each app, and connect its containers to it.

Quick checks when something fails

• docker ps shows whether the container is running and which ports it publishes.
• docker network inspect app-net lists the containers on that network.
• docker logs <name> shows what the app says when a connection is refused.