Connecting containers to multiple networks in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of docker network ls after creating two networks and connecting a container to both?

You create two Docker networks named net1 and net2. Then you run a container mycontainer connected to net1. Finally, you connect mycontainer to net2 using docker network connect. What will docker network ls show regarding these networks?

Docker

docker network create net1
docker network create net2
docker run -dit --name mycontainer --network net1 alpine sh
docker network connect net2 mycontainer
docker network ls

A Shows an error because a container cannot be connected to multiple networks.

B Lists only net1 network because the container was initially connected to net1.

C Lists both net1 and net2 networks with their IDs and drivers, showing both as active networks.

D Lists only net2 network because the container was connected last to net2.

💡 Hint

Think about how Docker networks are listed independently of container connections.

The docker network ls command lists all existing networks regardless of container connections. Creating two networks and connecting a container to both does not remove or hide any network. Both net1 and net2 will appear in the list.

🔀 Workflow

intermediate

Which sequence correctly connects a running container to two different networks?

You have a running container named webapp. You want to connect it to two networks: frontend and backend. Which sequence of commands will achieve this?

A

docker network create frontend
docker network create backend
docker network connect frontend webapp
docker network connect backend webapp

B

docker network create frontend
RUN docker network connect backend webapp
docker network connect frontend webapp

C

docker network connect frontend webapp
docker network create backend
docker network connect backend webapp

D

docker network create frontend
RUN docker network create backend
RUN docker network connect webapp frontend
RUN docker network connect webapp backend

💡 Hint

Remember to create networks before connecting containers to them.

You must create the networks first before connecting a container to them. The docker network connect command connects an existing container to an existing network. The correct sequence creates both networks first, then connects the container to each.

❓ Troubleshoot

advanced

Why does connecting a container to a network fail with "container is already connected to network" error?

You run docker network connect mynet mycontainer but get the error: container is already connected to network mynet. What is the most likely cause?

A The container is already connected to the network; trying to connect again causes this error.

B The network <code>mynet</code> does not exist, so Docker cannot connect the container.

C The container <code>mycontainer</code> is stopped, so it cannot be connected to a network.

D Docker daemon is not running, causing network commands to fail.

💡 Hint

Check if the container is already connected to the network before connecting again.

This error means the container is already connected to the specified network. Docker prevents duplicate connections to the same network for a container.

🧠 Conceptual

advanced

What happens to container network connectivity when it is connected to multiple Docker networks?

A container is connected to two Docker networks: netA and netB. How does this affect the container's network interfaces and communication?

A The container loses network connectivity because Docker does not support multiple networks per container.

B The container merges both networks into one interface, sharing IP addresses from both networks.

C The container can only communicate on the first network it was connected to; the second is ignored.

D The container gets multiple network interfaces, one per network, allowing communication on both networks independently.

💡 Hint

Think about how physical computers connect to multiple networks with multiple interfaces.

Docker assigns a separate network interface inside the container for each connected network. This allows the container to communicate on all connected networks independently.

✅ Best Practice

expert

Which practice is best when connecting containers to multiple networks in production?

In a production environment, you connect containers to multiple Docker networks for isolation and communication. Which practice is best to ensure security and manageability?

A Connect containers to multiple networks randomly to balance traffic automatically.

B Use user-defined bridge networks with clear naming and restrict container connections only to required networks.

C Connect all containers to the default bridge network and add extra networks as needed without restrictions.

D Use host networking mode for all containers to avoid network isolation complexities.

💡 Hint

Think about network isolation and clarity in production setups.

Using user-defined bridge networks allows better control, isolation, and naming clarity. Connecting containers only to networks they need reduces attack surface and complexity.