Container Networking

Advanced ⭐ 120 XP ⏱ 18 min #docker#networking

How containers get IPs, talk to each other, and reach the outside world.

📖Theory

Docker provides several network drivers:

bridge (default) — a private internal network; containers get IPs and can reach each other. On a user-defined bridge, Docker adds DNS so containers resolve each other by name.
host — the container shares the host’s network stack (no isolation, no port mapping)
none — no networking
overlay — spans multiple hosts (Swarm/clusters)

Port publishing (-p host:container) exposes a container port to the host. Containers on the same user-defined bridge talk directly by name; the default bridge requires --link (legacy) or IPs.

🌍Real-World Example

docker network create app
docker run -d --name db --network app postgres:16
docker run -d --name api --network app -e DB=db:5432 myapi
# 'api' reaches the database at host 'db' over the 'app' network

docker run -d -p 8080:80 --network app nginx   # publish to host:8080
docker network inspect app                     # see attached containers + IPs

✍️Hands-On Exercise

Create a user-defined bridge and attach two containers that talk by name.
Explain the difference between bridge and host networking.
Publish a container port to the host and verify it from a browser/curl.
When would you use an overlay network?

🧾Cheat Sheet▾

Driver / cmd	Purpose
bridge	Default private network
user-defined bridge	Adds name-based DNS
host	Share host network stack
none	No networking
overlay	Multi-host (clusters)
`-p host:container`	Publish a port
`docker network create/inspect`	Manage networks

💬Common Interview Questions▾

How do containers on a user-defined bridge find each other?

Docker’s built-in DNS resolves container names to their IPs on that network, so they connect by name (e.g. db:5432) without hardcoding addresses.

What's the difference between bridge and host networking?

Bridge gives the container its own isolated network namespace with a private IP and port mapping. Host shares the host’s network stack directly — no isolation and no port publishing needed.

📚Official Documentation

↗ Docker — Networking overview

📝 My notes on this topic

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