How to Map Port in Docker: Simple Guide with Examples

To map a port in Docker, use the -p option with docker run like -p hostPort:containerPort. This connects a port on your computer to a port inside the container, allowing access to container services from outside.

Syntax

The basic syntax to map ports in Docker is:

• docker run -p hostPort:containerPort imageName

Here, hostPort is the port on your computer, and containerPort is the port inside the Docker container.

bash

docker run -p 8080:80 nginx

Example

This example runs an Nginx web server container and maps port 8080 on your computer to port 80 inside the container. You can then open http://localhost:8080 in your browser to see the Nginx welcome page.

bash

docker run -d -p 8080:80 nginx

Output

a1b2c3d4e5f6g7h8i9j0

# Container runs in detached mode with port 8080 mapped to 80

Common Pitfalls

Common mistakes when mapping ports include:

• Using the same hostPort for multiple containers causes conflicts.
• Forgetting to expose the correct containerPort inside the container.
• Not using -p but only EXPOSE in Dockerfile, which does not publish ports to the host.

Always check if the host port is free and correctly map ports to avoid access issues.

bash

docker run -p 80:80 nginx
# This may fail if port 80 is already used on host

docker run -p 8080:80 nginx
# Correct way if port 80 is busy on host

Quick Reference

Option	Description	Example
-p hostPort:containerPort	Maps host port to container port	-p 8080:80
-d	Runs container in background (detached)	-d
EXPOSE port	Declares port inside Dockerfile (not published)	EXPOSE 80

Key Takeaways

Use -p hostPort:containerPort with docker run to map ports.

Host port must be free and not used by other services or containers.

EXPOSE in Dockerfile only documents ports; it does not publish them to host.

Use different host ports to run multiple containers simultaneously.

Check container logs or browser to verify port mapping works.