Module 4 Lesson 3: Container Networking Basics

By default, Docker containers are isolated. If you have a web-app container and a database container, they cannot see each other unless you specifically build a "Bridge" between them.

1. The Three Default Networks

When you install Docker, it creates three networks for you:

1. Bridge: The default. Good for single containers running on one host.
2. Host: The container shares the host machine's IP address. There is no isolation. (Fast but insecure).
3. None: No networking. The container is a "locked room."

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2. User-Defined Bridge Networks

The "Professional Way" is to create your own networks. Why? Because on the default Bridge, containers can only talk to each other via IP Addresses (which change every time you restart!).

On a User-Defined Network, Docker provides automatic DNS Service Discovery.

The Workflow:

1. Create a network:

   docker network create my-app-net
   

2. Attach containers to it:

   docker run --name db --network my-app-net -d postgres
   docker run --name web --network my-app-net -d my-web-app
   

3. Communication: Now, your web app can connect to the database using the hostname db instead of an IP address! Docker's internal DNS handles the translation.

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Visualizing the Process

graph TD
    Start[Input] --> Process[Processing]
    Process --> Decision{Check}
    Decision -->|Success| End[Complete]
    Decision -->|Retry| Process

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3. Exposing vs. Publishing Ports

• Expose (Internal): Declares that the container listens on a port. Only other containers on the same network can see it.
• Publish (External): Mapping a port to your host (using -p). This makes it available to the outside world (your browser).

Security Tip: Never publish your Database port (-p 5432:5432) to the outside world. Keep it "Exposed" only to the web-app on a private network.

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4. Useful Network Commands

• docker network ls: List all networks.
• docker network inspect <name>: See which containers are attached and their IP addresses.
• docker network connect <net> <container>: Add a running container to an existing network.

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Exercise: The Network Connectivity Test

1. Create a network called island-net.
2. Run two containers using the alpine image on that network: container-a and container-b.
   • Hint: docker run -d --name container-a --network island-net alpine sleep 1000
3. "Jump inside" container-a using exec.
4. Try to ping container-b. Does it work?
5. Try to ping google.com. Does it work? (If yes, why? Containers have outbound internet access by default).

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Summary

Networking is what turns isolated "Boxes" into a functioning "System." By using User-Defined Bridge networks, you gain the power of Service Discovery, allowing your apps to talk to each other by name rather than unpredictable IP addresses.

Next Lesson: Data that survives: Volumes and persistent storage.