Port mapping with -p flag in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the -p flag do in a Docker run command?

The -p flag maps a port on your computer (host) to a port inside the Docker container. This lets you access the container's service from outside.

beginner

How do you map port 8080 on your computer to port 80 inside a Docker container?

Use docker run -p 8080:80 <image-name>. This means requests to port 8080 on your computer go to port 80 in the container.

intermediate

Can you map multiple ports using the -p flag?

Yes, you can use multiple -p flags to map several ports, like -p 8080:80 -p 443:443.

intermediate

What happens if you try to map a host port that is already in use?

Docker will give an error because the port is busy. You must choose a free port on your computer to map.

advanced

Explain the difference between -p and --expose in Docker.

-p maps container ports to host ports for external access. --expose only makes ports available inside Docker networks but not to the host.

What does docker run -p 3000:80 myapp do?

A Maps host port 3000 to container port 80

B Maps container port 3000 to host port 80

C Runs container on port 3000 only

D Exposes port 80 inside the container only

The syntax -p hostPort:containerPort maps host port 3000 to container port 80.

Which command maps multiple ports in Docker?

A docker run -p 80:80,443:443 myapp

B docker run -port 80 443 myapp

C docker run -p 80:80 -p 443:443 myapp

D docker run --expose 80 443 myapp

Use multiple -p flags to map multiple ports.

If port 8080 on your computer is busy, what happens when you run docker run -p 8080:80 myapp?

A Docker shows an error and stops

B Docker runs but port 8080 is ignored

C Docker automatically picks a free port

D Docker maps to a random port

Docker cannot bind to a busy port and will show an error.

What is the correct order of ports in the -p flag?

A containerPort:hostPort

B hostPort:containerPort

C hostPort only

D containerPort only

The format is -p hostPort:containerPort.

Which flag exposes a container port only inside Docker networks but not to the host?

A -p

B -P

C --publish-all

D --expose

--expose makes ports available inside Docker networks but does not map them to the host.

Explain how port mapping works in Docker using the -p flag.

Think about how your computer talks to the container through ports.

Describe what happens if you try to map a host port that is already in use by another program.

Consider what your computer does when two apps want the same door.