🌐 Docker Networking

Containers don’t run in complete isolation forever — at some point they need to talk to each other, or to the outside world. That’s what Docker networking is all about. 🌐

🧠 How Does Container Networking Work?

When Docker runs a container, it creates a virtual network interface (eth0) inside that container. This interface connects to a virtual ethernet pair (veth) on the host, which then connects to the docker0 bridge — Docker’s built-in internal network switch.

That bridge then connects to the host’s real network interface (eth0), which reaches the internet.

Container Networking Diagram

Reading the diagram:

Each container has its own eth0 (virtual network card inside the container)
veth1 / veth2 → virtual cables connecting containers to the host
docker0 → Docker’s bridge, the internal switch that routes traffic between containers
eth0 (bottom) → the host machine’s real network card, connecting to the internet

🗂️ Container Networking Models

Docker gives you different network drivers depending on how you want containers to communicate.

Network Driver	What it does	Best for
bridge	Default. Containers on the same bridge can talk to each other. Isolated from host.	Most apps, development
host	Container shares the host’s network directly. No isolation.	Performance-sensitive apps
overlay	Connects containers across multiple Docker hosts (machines).	Docker Swarm / multi-server setups
none	No network at all. Complete isolation.	Maximum security needs

🌉 Bridge Network (Default)

When you run a container without specifying a network, it joins the default bridge network automatically.

docker run nginx

Containers on the same bridge network can communicate using their container IP addresses. However, containers on the default bridge cannot resolve each other by name — you need a custom bridge for that (covered below).

Check which network a container is on:

docker inspect <container-name>

🖥️ Host Network

With host networking, the container shares the host machine’s network stack directly. There’s no port mapping needed.

docker run --network host nginx

The downside: no isolation. The container can see everything on the host network. Use carefully.

☁️ Overlay Network

Overlay networks connect containers running on different machines (different Docker hosts). This is mainly used with Docker Swarm for distributed applications.

docker network create --driver overlay my-overlay

🛠️ Common Container Networking Commands

Docker Networking Commands

Here’s what each command does:

# List all networks on your machine
docker network ls

# Inspect a network (see which containers are on it, subnet, etc.)
docker network inspect <network-name>

# Create a new custom network
docker network create <network-name>

# Run a container on a specific network
docker run --network my-network <image>

# Connect a running container to a network
docker network connect my-network <container>

# Disconnect a container from a network
docker network disconnect my-network <container>

# Delete a network
docker network rm my-network

🔧 Create a Custom Network and Run Containers on It

This is where it gets powerful. Custom bridge networks let containers talk to each other by name (DNS resolution built in).

Step 1 — Create the network:

docker network create my-network

Step 2 — Run containers on it:

docker run -d --name app --network my-network node:22-alpine
docker run -d --name db --network my-network mongo

Step 3 — Containers can now ping each other by name:

# From inside the app container, you can reach the db by name
ping db

This works because Docker’s built-in DNS resolves container names automatically on custom networks. On the default bridge, this doesn’t work — you’d need to use IP addresses directly.

💡 Best practice: Always create a custom network for your apps instead of using the default bridge. It’s cleaner and gives you name-based DNS for free.

✅ Quick Summary

Task	Command
List networks	`docker network ls`
Inspect a network	`docker network inspect <name>`
Create a network	`docker network create <name>`
Run on a network	`docker run --network <name> <image>`
Connect to network	`docker network connect <name> <container>`
Disconnect from network	`docker network disconnect <name> <container>`
Delete a network	`docker network rm <name>`

The 3 drivers you’ll use most:

bridge → default, isolated, one machine
host → no isolation, shares host network
overlay → multiple machines (Swarm)

📚 Further Reading

Docker Networking Overview — Official Docker Docs
Bridge Network Driver — How the default bridge works
Overlay Network Driver — For multi-host setups