Port mapping in Compose in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is port mapping in Docker Compose?

Port mapping connects a port on your computer (host) to a port inside a Docker container, allowing you to access container services from outside.

beginner

How do you define port mapping in a docker-compose.yml file?

Use the ports key under a service with the format host_port:container_port. For example:

ports:
  - "8080:80"

maps host port 8080 to container port 80.

beginner

Why is port mapping important when running containers?

Containers run isolated with their own network. Port mapping lets you reach container apps from your computer or other devices by linking container ports to host ports.

intermediate

What happens if you map the same host port to multiple containers in Compose?

You get a conflict error because only one container can use a host port at a time. You must use different host ports for each container.

intermediate

How do you expose a container port without mapping it to a host port in Compose?

Use the expose key instead of ports. This makes the port accessible to linked containers but not to the host machine.

In docker-compose.yml, how do you map host port 5000 to container port 80?

A ports: - "80:5000"

B expose: - "5000:80"

C ports: - "5000:80"

D expose: - "80:5000"

The correct format is host_port:container_port. So 5000:80 maps host port 5000 to container port 80.

What does the expose key do in docker-compose?

A Makes container ports accessible only to linked containers

B Maps container ports to host ports

C Blocks all container ports

D Maps host ports to container ports

expose shares ports between containers but does not publish them to the host.

What error occurs if two services map the same host port in Compose?

A No error, ports are shared

B Port conflict error

C Container crash without error

D Docker ignores the second mapping

Docker Compose throws a port conflict error because only one container can bind to a host port.

Which port is accessible from your computer when you map 8080:80?

A Port 80 inside the container only

B Port 8080 inside the container

C Port 80 on your computer

D Port 8080 on your computer

Host port 8080 is accessible from your computer and forwards traffic to container port 80.

If you want a container port accessible only to other containers, which should you use?

A expose

B networks

C volumes

D ports

expose shares ports between containers without exposing them to the host.

Explain how port mapping works in Docker Compose and why it is useful.

Think about how your computer talks to apps inside containers.

Describe the difference between the ports and expose keys in a Compose file.

One lets outside world in, the other only shares inside Docker.