Port mapping with -p flag in Docker - Mini Project: Build & Apply

Port mapping with -p flag

📖 Scenario: You want to run a web server inside a Docker container and make it accessible from your computer's browser.To do this, you need to map the container's internal port to a port on your computer.

🎯 Goal: Learn how to use the -p flag in the docker run command to map ports from the container to your computer.

📋 What You'll Learn

Create a Docker container running the nginx web server.

Map the container's port 80 to your computer's port 8080 using the -p flag.

Verify the port mapping by running the container and checking the output.

💡 Why This Matters

🌍 Real World

Port mapping is essential when running web servers or services inside containers so you can access them from your computer or network.

💼 Career

Understanding port mapping helps you deploy containerized applications and troubleshoot network access issues in real environments.

1

Create a Docker container running nginx

Write the docker run command to start a container named mynginx using the nginx image without any port mapping.

Docker

# Write the docker run command to start nginx container named mynginx
# Your code here

Hint

Use docker run --name mynginx -d nginx to start the container in detached mode.

2

Add port mapping with -p flag

Modify the docker run command to map port 8080 on your computer to port 80 inside the container using the -p flag.

Docker

docker run --name mynginx -d nginx
# Add the -p flag to map port 8080 on host to port 80 in container
# Your code here

Hint

Use -p 8080:80 to map ports.

3

Run the container with port mapping

Run the docker run command with the -p 8080:80 flag and container name mynginx to start the nginx server with port mapping.

Docker

docker run --name mynginx -d -p 8080:80 nginx
# Run the container with port mapping
# Your code here

Hint

Use the same command as before to run the container.

4

Verify port mapping with docker ps

Write the docker ps command to list running containers and show the port mapping for mynginx.

Docker

docker run --name mynginx -d -p 8080:80 nginx
# Write the docker ps command to verify port mapping
# Your code here

Hint

Use docker ps to see running containers and their port mappings.

Practice

1.

What does the -p flag do in the docker run command?

easy

A. Names the container

B. Sets the container's CPU limit

C. Maps a port on the host to a port in the container

D. Specifies the container's environment variables

Solution

1. Step 1: Understand the purpose of the -p flag

   The -p flag is used to connect a port on your computer (host) to a port inside the Docker container.

2. Step 2: Identify what port mapping means

   This mapping allows you to access services running inside the container from your host machine using the specified port.

3. Final Answer:

   Maps a port on the host to a port in the container -> Option C

4. Quick Check:

   -p flag = port mapping [OK]

Hint: Remember: -p connects host port to container port [OK]

Common Mistakes:

• Confusing -p with CPU or memory limits
• Thinking -p sets container name
• Assuming -p sets environment variables

2.

Which of the following is the correct syntax to map host port 8080 to container port 80?

docker run -p ??? image_name

easy

A. -p 8080:80

B. -p 80:8080

C. -p 8080-80

D. -p 80-8080

Solution

1. Step 1: Recall the port mapping format

   The correct format is -p host_port:container_port, so the host port comes first.

2. Step 2: Apply the ports given

   Host port is 8080, container port is 80, so it should be -p 8080:80.

3. Final Answer:

   -p 8080:80 -> Option A

4. Quick Check:

   Host port first, container port second [OK]

Hint: Host port comes before container port in -p [OK]

Common Mistakes:

• Swapping host and container ports
• Using dash (-) instead of colon (:)
• Using invalid port range syntax

3.

What will happen if you run this command?
docker run -p 5000:80 nginx
Then open http://localhost:5000 in your browser?

medium

A. Nothing happens because port 5000 is not mapped

B. You will see the default Nginx welcome page

C. You will see a Docker error about port mapping

D. Connection refused error because port 80 is blocked

Solution

1. Step 1: Understand the port mapping

   The command maps host port 5000 to container port 80 where Nginx serves its default page.

2. Step 2: Accessing the service

   Opening http://localhost:5000 accesses the container's port 80 service, showing Nginx's default page.

3. Final Answer:

   You will see the default Nginx welcome page -> Option B

4. Quick Check:

   Port 5000 maps to Nginx port 80, page loads [OK]

Hint: Host port maps to container port, access via host port [OK]

Common Mistakes:

• Thinking port 80 on host is used
• Assuming port mapping causes errors
• Not realizing Nginx serves on port 80 inside container

4.

What is wrong with this command?
docker run -p 80:8080 myapp

The app inside the container listens on port 8080.

medium

A. Host port 80 is mapped to container port 8080, which is correct

B. The ports are reversed; it should be -p 8080:80

C. The container port should be 80 if the app listens on 8080

D. Nothing is wrong; this command works as expected

Solution

1. Step 1: Check the app's listening port

   The app listens on container port 8080, so container port must be 8080 in mapping.

2. Step 2: Verify the port mapping syntax

   The command maps host port 80 to container port 8080, which is correct to expose app on host port 80.

3. Final Answer:

   Host port 80 is mapped to container port 8080, which is correct -> Option A

4. Quick Check:

   Host:container = 80:8080 is valid [OK]

Hint: Remember: host port first, container port second [OK]

Common Mistakes:

• Swapping host and container ports
• Assuming container port must be 80
• Confusing app listening port with host port

5.

You want to run two containers from the same image that both expose port 80 inside the container. How can you map their ports so both are accessible on your host machine?

hard

A. You cannot run two containers exposing the same port

B. Use -p 80:80 for both containers

C. Use -p 80:8080 for the first and -p 80:8081 for the second

D. Use -p 8080:80 for the first and -p 8081:80 for the second container

Solution

1. Step 1: Understand port conflicts on host

   Two containers cannot map the same host port (like 80) simultaneously because the host port must be unique.

2. Step 2: Assign different host ports

   Map the first container's port 80 to host port 8080, and the second container's port 80 to host port 8081 to avoid conflicts.

3. Final Answer:

   Use -p 8080:80 for the first and -p 8081:80 for the second container -> Option D

4. Quick Check:

   Different host ports avoid conflicts [OK]

Hint: Use different host ports for multiple containers [OK]

Common Mistakes:

• Trying to map both containers to host port 80
• Swapping host and container ports
• Thinking containers can share the same host port