EXPOSE instruction for ports in Docker - Step-by-Step Execution

Process Flow - EXPOSE instruction for ports

Write Dockerfile with EXPOSE

↓

Build Docker Image

↓

Run Container

↓

Port is marked as exposed

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Optional: Map port to host

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Container can accept traffic on exposed port

This flow shows how the EXPOSE instruction marks a port inside the container as open for communication, which can then be mapped to the host when running the container.

Execution Sample

Docker

FROM nginx:latest
EXPOSE 80
CMD ["nginx", "-g", "daemon off;"]

This Dockerfile exposes port 80 so the container can receive web traffic on that port.

Process Table

Step	Action	Result	Notes
1	Dockerfile read: FROM nginx:latest	Base image set to nginx	Sets base image
2	Dockerfile read: EXPOSE 80	Port 80 marked as exposed	Port 80 is now known as open inside container
3	Dockerfile read: CMD ["nginx", "-g", "daemon off;"]	Default command set	Starts nginx server
4	Build image	Image created with exposed port 80	EXPOSE info stored in image metadata
5	Run container without port mapping	Container runs, port 80 exposed internally	Port 80 not accessible from host by default
6	Run container with -p 8080:80	Port 80 in container mapped to 8080 on host	Host can access container on localhost:8080
7	Send HTTP request to localhost:8080	Request reaches nginx inside container	Port mapping works
8	Stop container	Container stops	End of lifecycle

💡 Container stopped, port exposure and mapping demonstrated

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
EXPOSED_PORTS	[]	[80]	[80]	[80]	[80]
CONTAINER_RUNNING	false	false	false	true	false
PORT_MAPPING	{}	{}	{}	{8080:80}	{8080:80}

Key Moments - 2 Insights

Does EXPOSE alone make the port accessible from the host machine?

No, EXPOSE only marks the port inside the container as open. To access it from the host, you must map the port using -p or --publish when running the container, as shown in step 6 of the execution table.

What happens if you run the container without port mapping?

The container runs and the port is exposed internally, but the host cannot access it. This is shown in step 5 where the container runs but port 80 is not accessible from outside.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the port 80 marked as exposed inside the image?

A Step 4

B Step 2

C Step 5

D Step 6

💡 Hint

Check the 'Action' column for when EXPOSE 80 is read in the Dockerfile.

According to the variable tracker, what is the value of PORT_MAPPING after step 4?

A {8080:80}

B {80:80}

C {}

D []

💡 Hint

Look at the PORT_MAPPING row and the After Step 4 column.

If you want to access the container's port 80 on your host's port 3000, which step in the execution table would change?

A Step 6

B Step 5

C Step 7

D Step 8

💡 Hint

Port mapping happens when running the container, see step 6.

Concept Snapshot

EXPOSE <port> in Dockerfile marks a port inside the container as open.
It does NOT publish the port to the host.
Use docker run -p hostPort:containerPort to map ports.
EXPOSE helps document which ports the container listens on.
Mapping ports enables host access to container services.

Full Transcript

The EXPOSE instruction in a Dockerfile tells Docker that the container listens on the specified network port at runtime. When building the image, this port is marked as exposed in the image metadata. However, this does not automatically make the port accessible from the host machine. To allow access, you must map the container port to a host port using the -p option when running the container. For example, running 'docker run -p 8080:80' maps container port 80 to host port 8080. Without this mapping, the port is only open inside the container and cannot be reached from outside. This visual execution showed reading the Dockerfile, building the image, running the container with and without port mapping, and how the port exposure and mapping affect accessibility.