Port mapping with -p flag in Docker - Commands & Configuration

Introduction

A container runs in its own isolated network and its ports are not accessible from your computer by default. The -p flag maps a port on your host machine to a port inside the container so you can reach the app running inside it.

When you run a web server in a container and want to open it in your browser on your computer.

When you run a database container and want your app on the host to connect to it.

When you need to expose multiple services from the same container on different host ports.

When you want to test a containerized API from tools like Postman or curl on your machine.

When you run multiple containers with the same internal port and need to map each to a different host port.

Commands

Runs an nginx container in detached mode and maps port 8080 on the host to port 80 inside the container. Opening http://localhost:8080 in your browser will show the nginx welcome page.

Terminal

docker run -d -p 8080:80 nginx

Expected Output

a1b2c3d4e5f67890abcdef1234567890abcdef1234567890abcdef1234567890

→

-p 8080:80 - Maps host port 8080 to container port 80 (format: host:container)

→

-d - Runs the container in the background

Lists running containers to confirm the port mapping is active and the container is healthy.

Terminal

docker ps

Expected Output

CONTAINER ID   IMAGE   COMMAND                  CREATED         STATUS         PORTS                  NAMES

a1b2c3d4e5f6   nginx   "/docker-entrypoint.…"   5 seconds ago   Up 4 seconds   0.0.0.0:8080->80/tcp   relaxed_morse

Shows all port mappings for the container, confirming which host ports are forwarded to which container ports.

Terminal

docker port relaxed_morse

Expected Output

80/tcp -> 0.0.0.0:8080

Key Concept

If you remember nothing else from this pattern, remember: the -p flag format is always host_port:container_port — left side is your machine, right side is inside the container.

Common Mistakes

Reversing the port order to container:host instead of host:container

Docker maps the wrong direction — the container port becomes the host port, causing connection failures.

Always write -p host_port:container_port, e.g. -p 8080:80 to reach container port 80 via host port 8080.

Using a host port already in use by another process

Docker fails to start the container with a 'port is already allocated' error.

Choose a free host port or run 'lsof -i :PORT' to check if a port is in use before mapping.

Forgetting to publish ports and wondering why the container is unreachable

Without -p, the container port is only accessible inside the Docker network, not from the host.

Always add -p host:container when you need to access a containerized service from outside.

Summary

Use -p host_port:container_port to expose a container service to your host machine.

The PORTS column in 'docker ps' shows 0.0.0.0:8080->80/tcp confirming the mapping is active.

Use 'docker port container_name' to inspect all active port mappings for a running container.

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