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Day 4: Docker Networking - Fundamentals & Host Communication

Concepts:

  • Container Networking: How Docker containers communicate with each other and with the outside world.
  • Isolated Network Stack: Each Docker container by default gets its own isolated network stack (IP address, routing table, DNS).
  • Bridge Network (Default): The most common network type. Containers on the same bridge network can communicate by IP address. Docker automatically creates a default bridge network.
  • Host Network: Container shares the host's network stack. No network isolation between the container and the host.
  • None Network: Disables all networking for the container.
  • Port Mapping (-p or --publish): The mechanism to map a port from the container's network stack to a port on the Docker host's network stack, allowing external access.

Examples:

  1. Inspect Docker's default networks:
docker network ls
docker network inspect bridge

(Note the Subnet and Gateway of the bridge network.)

  1. Run a simple web server on the default bridge network (without port mapping):
docker run -d --name nginx_internal nginx:latest
docker inspect nginx_internal | grep "IPAddress"

(You won't be able to access this Nginx from your host browser because no port is mapped.)

  1. Run Nginx with port mapping (default bridge network):
docker run -d -p 8080:80 --name nginx_web nginx:latest

(Access http://localhost:8080 in your browser. This maps host port 8080 to container port 80.)

  1. Explore host networking:
docker run -it --network host alpine:latest ip addr show eth0

(The container's IP address will be the same as your host's primary IP address. Be careful, as this removes network isolation.)

  1. Explore none networking:
docker run -it --network none alpine:latest ip addr show

(You'll see no network interfaces configured other than the loopback.)

Challenge 4: Port Mapping & Container Reachability

  • Run your Flask application from Day 2 in a container, but this time, map its internal port (e.g., 5000) to two different host ports (e.g., 8081 and 8082). Can you access it from both host ports?
  • Run two separate Nginx containers, both exposing port 80 internally, but map them to different host ports (e.g., 8083 and 8084). Verify you can reach both.
  • Try to run a container using --network host that starts a web server on port 80. What happens if your host machine already has a web server running on port 80?