Docker Networks

10. Docker Networks

🌐 What is Docker Networking?

By default, containers are isolated. Docker networking lets containers communicate with each other and with the outside world.

Analogy: Think of Docker networks like office floors. Containers on the same network (floor) can talk freely. Containers on different networks (floors) can’t talk unless you set up a connection.

🕸️ Network Types

Network Driver	Description	Use Case
`bridge`	Default, isolated network	Single-host container communication
`host`	Container shares host network	High performance, Linux only
`none`	No networking	Maximum isolation
`overlay`	Multi-host networking	Docker Swarm, Kubernetes
`macvlan`	Container gets own MAC address	Legacy apps needing direct network access

🗺️ Network Diagram

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  <text x="350" y="70" text-anchor="middle" font-size="13">🌍 Internet</text>

  <!-- Host Machine -->
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  <!-- Bridge Network -->
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  <text x="350" y="170" text-anchor="middle" font-size="11" font-weight="bold" fill="#2e7d32">my-bridge-network (172.18.0.0/16)</text>

  <!-- Container 1: Web App -->
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  <text x="175" y="218" text-anchor="middle" font-size="10" fill="#388e3c">Node.js API</text>
  <text x="175" y="234" text-anchor="middle" font-size="10" fill="#555">IP: 172.18.0.2</text>
  <text x="175" y="250" text-anchor="middle" font-size="10" fill="#555">Port: 3000</text>
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  <text x="175" y="277" text-anchor="middle" font-size="10" fill="#1b5e20">-p 3000:3000</text>

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  <text x="355" y="202" text-anchor="middle" font-size="12" font-weight="bold" fill="#1b5e20">mongodb</text>
  <text x="355" y="218" text-anchor="middle" font-size="10" fill="#388e3c">MongoDB</text>
  <text x="355" y="234" text-anchor="middle" font-size="10" fill="#555">IP: 172.18.0.3</text>
  <text x="355" y="250" text-anchor="middle" font-size="10" fill="#555">Port: 27017</text>
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  <text x="355" y="277" text-anchor="middle" font-size="10" fill="#1b5e20">no external port</text>

  <!-- Container 3: Redis -->
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  <text x="530" y="234" text-anchor="middle" font-size="10" fill="#555">IP: 172.18.0.4</text>
  <text x="530" y="250" text-anchor="middle" font-size="10" fill="#555">Port: 6379</text>
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  <text x="530" y="277" text-anchor="middle" font-size="10" fill="#1b5e20">internal only</text>

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  <text x="370" y="108" font-size="9" fill="#1565c0">:3000 exposed</text>
</svg>

🔧 Network Commands

# List all networks
docker network ls

# Create a custom bridge network
docker network create my-network

# Create with custom subnet
docker network create --subnet=172.20.0.0/16 my-network

# Run container on specific network
docker run --network my-network --name web nginx

# Connect existing container to a network
docker network connect my-network my-container

# Disconnect from network
docker network disconnect my-network my-container

# Inspect network details
docker network inspect my-network

# Remove unused networks
docker network prune

# Remove specific network
docker network rm my-network

📡 Container-to-Container Communication

On the same custom network, containers can reach each other by name:

# Create network
docker network create app-network

# Start MongoDB
docker run -d --name mongodb --network app-network mongo

# Start Node.js app — it can connect to "mongodb" by name!
docker run -d \
  --name api \
  --network app-network \
  -e MONGO_URL=mongodb://mongodb:27017/mydb \
  my-node-app

# Inside the API container, use: mongodb://mongodb:27017
# Docker's DNS resolves "mongodb" to the container's IP automatically

💡 Key Insight: Use custom networks (not the default bridge). On custom networks, Docker automatically provides DNS resolution by container name.