Port mapping with -p flag in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Understanding basic port mapping with Docker -p flag

What is the output when you run the command docker run -d -p 8080:80 nginx and then check the port mapping with docker port <container_id>?

A 80/tcp -> 0.0.0.0:8080

B 8080/tcp -> 127.0.0.1:80

C 80/tcp -> 127.0.0.1:8080

D 8080/tcp -> 0.0.0.0:80

💡 Hint

Remember the format is :.

The -p 8080:80 flag maps port 8080 on the host to port 80 inside the container. The docker port command shows this mapping as 80/tcp -> 0.0.0.0:8080.

🧠 Conceptual

intermediate

Effect of omitting host IP in port mapping

When using docker run -p 127.0.0.1:8080:80 nginx, what is the difference compared to docker run -p 8080:80 nginx?

A The container port 80 is accessible from any IP on the host network.

B The container port 80 is only accessible from localhost (127.0.0.1) on the host.

C The container port 80 is not accessible at all.

D The container port 80 is mapped to port 127.0.0.1 on the container.

💡 Hint

Think about binding to a specific IP address on the host.

Specifying 127.0.0.1:8080:80 binds the port only to localhost on the host machine, so it is not accessible from other machines on the network.

❓ Troubleshoot

advanced

Diagnosing port conflict error with Docker -p flag

You run docker run -p 80:80 nginx but get an error: Bind for 0.0.0.0:80 failed: port is already allocated. What is the most likely cause?

A Another process on the host is already using port 80.

B The container image does not expose port 80.

C Docker daemon is not running.

D The container port 80 is blocked by firewall inside the container.

💡 Hint

Check what is using port 80 on your host machine.

The error means the host port 80 is busy, so Docker cannot bind the container port 80 to it. You can check with sudo lsof -i :80 or similar.

🔀 Workflow

advanced

Correct order to expose and map ports in Dockerfile and run command

Which is the correct order of steps to expose port 5000 in a Dockerfile and map it to host port 3000 when running the container?

A 2,1,3,4

B 3,1,2,4

C 1,3,2,4

D 1,2,3,4

💡 Hint

Think about building the image after editing the Dockerfile.

You first add EXPOSE in the Dockerfile, then build the image, then run the container with port mapping, and finally verify the mapping.

✅ Best Practice

expert

Choosing the safest port mapping for a public web app

You want to run a web app in Docker accessible from the internet on port 80, but avoid exposing the container port directly to all interfaces. Which command is the safest choice?

A docker run -p 0.0.0.0:80:80 mywebapp

B docker run -p 127.0.0.1:80:80 mywebapp

C docker run -p 192.168.1.100:80:80 mywebapp

D docker run -p 80:80 mywebapp

💡 Hint

Consider binding to a specific public IP address of your host.

Binding to 192.168.1.100 (a specific host IP) exposes the app only on that interface, reducing risk compared to binding to all interfaces (0.0.0.0). Binding to localhost (127.0.0.1) would block external access.