Exploring Bridge Networks in Docker
In this documentation, we'll explore bridge networks in Docker by creating and examining custom bridge networks, attaching containers to multiple networks, and using tools like ip and nmap to examine network configurations and discover other containers.

Creating and Examining a Custom Bridge Network
Let's start by creating a custom bridge network named user-network and exploring its configuration:

docker network create \
  --driver bridge \
  --label project=dockerinaction \
  --label chapter=5 \
  --attachable \
  --scope local \
  --subnet 10.0.42.0/24 \
  --ip-range 10.0.42.128/25 \
  user-network

This Docker command creates a custom bridge network with the following configurations:

Bridge driver for local container communication.
Labels (project=dockerinaction, chapter=5) to categorize and organize the network.
Attachable network allows standalone containers to join.
Scope: Local, restricting the network to a single Docker host.
Custom subnet (10.0.42.0/24) and IP range (10.0.42.128/25), controlling the IPs assigned to containers.
Network name: user-network.
This setup allows precise control over container networking and IP management within the defined range.

Inspect the newly created bridge:

Now, let's launch a container attached to this network and examine its network interfaces:

docker run -it \
  --network user-network \
  --name network-explorer \
  alpine:3.8 \
    sh

Inside the container, run the following command to list the available IPv4 addresses:

ip -f inet -4 -o addr

We'll see output similar to:

This output shows that the container has an IP address within the range and subnet specified by the user-network configuration.

Attaching Containers to Multiple Networks
Now, let's attach the network-explorer container to another bridge network named user-network2:

docker network create \
  --driver bridge \
  --label project=dockerinaction \
  --label chapter=5 \
  --attachable \
  --scope local \
  --subnet 10.0.43.0/24 \
  --ip-range 10.0.43.128/25 \
  user-network2

To see the networks :

docker network ls

Expected Output:

Next, attach the network-explorer container to user-network2:

docker network connect \
  user-network2 \
  network-explorer

Installing nmap Inside the Container
Inside the network-explorer container, install the nmap package:

docker start network-explorer

docker exec -it network-explorer sh -c "apk update && apk add nmap"

This command executes a shell within the network-explorer container and installs the nmap package.

nmap: Installing nmap inside a Docker container provides network administrators and developers with a versatile tool for network exploration, security auditing, troubleshooting, and monitoring within the containerized environment.

Using nmap to Explore the Network
Now, scan the subnets of both bridge networks:

nmap -sn 10.0.42.* -sn 10.0.43.* -oG /dev/stdout | grep Status

Ping Scan: nmap is used to perform a ping scan (-sn) on IP ranges 10.0.42.* and 10.0.43.*.

Output Filtering: Results are formatted in a greppable format (-oG /dev/stdout) and filtered using grep to display only lines containing "Status".

We'll see output indicating the status of devices on each subnet, including containers and gateway adapters.

Conclusion
Exploring bridge networks in Docker allows us to understand network configurations, attach containers to multiple networks, and use tools like ip and nmap to examine network settings and discover other containers. This knowledge is essential for managing containerized applications and designing network architectures in Docker environments.