Port mapping with -p flag in Docker - Step-by-Step Execution

Process Flow - Port mapping with -p flag

Start Docker Container

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Use -p flag: hostPort:containerPort

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Docker maps hostPort to containerPort

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Host can access container service via hostPort

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Container listens on containerPort internally

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End

This flow shows how Docker uses the -p flag to connect a port on your computer (host) to a port inside the container, allowing access to container services.

Execution Sample

Docker

docker run -p 8080:80 nginx

Runs an nginx container mapping host port 8080 to container port 80.

Process Table

Step	Command Part	Action	Result	Notes
1	docker run	Start container	Container created and started	Base command to run container
2	-p 8080:80	Map ports	Host port 8080 linked to container port 80	Enables access from host to container service
3	nginx	Select image	nginx image pulled (if needed) and used	Container runs nginx server
4	Container running	Listen on port 80	nginx listens inside container on port 80	Internal container port
5	Host access	Connect to localhost:8080	Request forwarded to container port 80	User accesses nginx via host port
6	Exit	Stop container	Container stops	End of container lifecycle

💡 Container stops or user interrupts; port mapping ends with container lifecycle

Status Tracker

Variable	Start	After Step 2	After Step 4	Final
hostPort	none	8080	8080	8080
containerPort	none	80	80	80
containerState	not running	running	running	stopped

Key Moments - 3 Insights

Why do we write -p 8080:80 and not just -p 80?

The execution_table row 2 shows that -p 8080:80 maps host port 8080 to container port 80. Without specifying host port, Docker assigns a random port on the host, which may be hard to access.

Does the container listen on port 8080 inside?

No, as shown in execution_table row 4, the container listens on port 80 internally. Port 8080 is only on the host side.

What happens if the host port 8080 is already in use?

Docker will fail to start the container with an error because it cannot bind host port 8080. This is implied by the port mapping step in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution_table, what port does the container listen on internally?

A 8080

B 443

C 80

D None

💡 Hint

Check Step 4 in execution_table where container listens on port 80

At which step is the host port 8080 linked to the container port?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look at Step 2 in execution_table where -p 8080:80 is processed

If you change the command to -p 9090:80, what changes in variable_tracker?

A hostPort changes to 9090

B containerPort changes to 9090

C containerState changes to 9090

D No changes

💡 Hint

See variable_tracker hostPort value after Step 2

Concept Snapshot

docker run -p hostPort:containerPort image
- Maps a port on your computer (hostPort) to a port inside the container (containerPort).
- Allows accessing container services via localhost:hostPort.
- Container listens internally on containerPort.
- Host port must be free to bind.
- Example: docker run -p 8080:80 nginx

Full Transcript

This visual execution shows how Docker's -p flag maps a port from your computer to a container. When you run 'docker run -p 8080:80 nginx', Docker starts an nginx container listening on port 80 inside. The host port 8080 is linked to container port 80, so accessing localhost:8080 reaches nginx. The execution table traces each step: starting container, mapping ports, running nginx, and stopping container. Variables track hostPort and containerPort values. Key moments clarify why both ports are needed and what happens if ports conflict. The quiz tests understanding of port mapping steps and variable changes. This helps beginners see how Docker connects host and container ports clearly.