Access Applications Using Docker Port Mapping

When an application runs inside a container, it is isolated from your host machine.

Even if the application listens on a port inside the container, your browser cannot automatically access it. You need to map a host port to a container port.

This is called port mapping.

Container port vs host port

A container has its own network space. An application may listen on port 80, 8080, or 3000 inside the container.

Your laptop or server also has its own ports.

Port mapping connects the two:

host port -> container port

For example:

docker run -p 8080:80 nginx

This means:

Now you can open:

http://localhost:8080

Docker sends that traffic to port 80 inside the container.

Run Nginx with port mapping

Use detached mode and map the port:

docker run -d -p 8080:80 --name web nginx

Check the container:

docker ps

In the ports column, you should see something like:

0.0.0.0:8080->80/tcp

That tells you host port 8080 is connected to container port 80.

Why not just use the same port

You can map the same port:

docker run -d -p 80:80 nginx

But during practice, ports like 8080 are often safer because port 80 may already be used or may require extra permissions on some systems.

You can also run multiple containers from the same image by using different host ports:

docker run -d -p 8081:80 nginx
docker run -d -p 8082:80 nginx

Both containers use port 80 internally, but your host exposes them on different ports.

Common mistakes

Port mapping mistakes are very common.

Check these things when the browser does not open your app:

• Is the container running?
• Did you use -p while starting it?
• Are the host and container ports in the correct order?
• Is the application listening inside the container?
• On AWS, is the security group allowing the host port?

Remember the order:

-p host-port:container-port

Stop and remove the container

After practice:

docker stop web
docker rm web

Written By: Shiva Srivastava