Demonstration of How to Do Port Mapping in Docker

STEP 1: Run NGINX on Port 8080

docker run -d --name mynginx -p 8080:80 nginx

NGINX is running inside container on port 80✔ You are accessing it from host on 8080

Test:

http://localhost:8080

STEP 2: Try to Reassign the SAME HOST PORT (8080) to Another Container

Now run:

docker run -d --name mynginx2 -p 8080:80 nginx

You will get an error:

Error starting userland proxy: listen tcp4 0.0.0.0:8080: bind: address already in use

Meaning:

• Port 8080 on your machine is already occupied by mynginx

• Docker cannot assign the same host port twice

This demonstrates port conflict.

STEP 3: Check Which Container Is Using Port 8080

Run:

docker ps

You will see:

0.0.0.0:8080->80/tcp   mynginx

STEP 4: Free Port 8080

Stop the container:

docker stop mynginx

Remove the container:

docker rm mynginx

Port 8080 is now free.

STEP 5: Run New Container Again on Port 8080

docker run -d --name mynginx2 -p 8080:80 nginx

✔ This time it works✔ Because port 8080 is free

Test:

http://localhost:8080

STEP 6: Reassign to a NEW PORT Instead (Optional)

If the old port is busy, assign another port:

docker run -d --name mynginx3 -p 9000:80 nginx

Test:

http://localhost:9000

What You Have Demonstrated

Concept	Explanation
Port Binding	-p HOST:CONTAINER
Port Conflict	Same host port cannot be used twice
Port Release	Stop/remove container to free port
Port Reassignment	Assign a new port like 9000