3.2: Focus: EXPOSE vs PORTS. Why is EXPOSE not enough to access my site?

WhyEXPOSEisn’t enough to access my site?

Now that we’ve covered port mapping, it’s important to understand the difference between two concepts that are often confused:

• EXPOSE in the Dockerfile
• -p or--publish docker run

EXPOSE : a simple declaration

EXPOSE is used to specify which port the container uses.

Example in a Dockerfile:

FROM nginx
EXPOSE 80

• This does not create an external redirect.
• It’s a kind of documentation forDocker and for developers.

Think ofEXPOSE as a sticky note on the door: “This container is listening on port 80.”
But the sticky note doesn’t send the signal outside.

-p/--publish : the actual port mapping

To make your site accessible from your machine or the Internet, you need to map the internal port to an external port using:

docker run -p PORT_EXTERNE:PORT_INTERNE image

Concrete example with Nginx:

docker run -p 8080:80 nginx

• 8080 → your machine
• 80 → container’s internal port

Without this step, even if the Dockerfile contains EXPOSE 80, no one will be able to access your site.

Quick comparison

Practical example

Dockerfile:

FROM nginx
EXPOSE 80

• The container listens on port 80.
• ButEXPOSE alone does not make the port accessible from your machine.

Start command without mapping:

docker run nginx

• The container starts, port 80 exists inside it, but
  you cannot access Nginx from your browser (http://localhostdoes not work).

Start command with port mapping:

docker run -p 8080:80 nginx

• Docker Redirects port 8080 on your machine to port 80 on the container.
• You can now open your browser to:

http://localhost:8080

• And access your Nginx site.

Simplified diagram

Votre machine           Conteneur Docker
---------------         -----------------
localhost:8080   --->   nginx:80
(port externe)          (port interne)

• Without-p: no arrow → inaccessible
• With-p: arrow to the container → accessible

EXPOSE simply indicates that the port exists;
-p it performs the actual redirection.