The port concept

Learn the relationship between host and container ports.

The port concept

Each container has its own network stack. nginx listening on port 80 inside the container doesn't automatically open port 80 on the host; it can't be reached from outside. For access, the container port is published to a port on the host (port mapping):

host:8080  ──►  container:80
docker run -d -p 8080:80 nginx
curl http://localhost:8080      # the nginx welcome page

Seeing published ports

The PORTS column of docker ps and the docker port command show the published mappings:

$ docker ps --format '{{.Names}} {{.Ports}}'
web 0.0.0.0:8080->80/tcp
$ docker port web
80/tcp -> 0.0.0.0:8080

0.0.0.0 means it listens on all of the host's network interfaces: another computer on the same network can reach it too.

Exercise Publish a web server

Write two commands: start a container named web from nginx in the background, mapping host port 8080 to container port 80; then show its port mappings.

Show example solution

docker run -d --name web -p 8080:80 nginx
docker port web

Test yourself

Does writing EXPOSE 80 in the Dockerfile provide access from outside?
- Only on Linux
- Only for port 80
- No; it only documents, -p is needed to publish
- Yes, always
Show answer

No; it only documents, -p is needed to publish — EXPOSE also tells which ports the -P flag will publish.
Show the container's port mappings.
```
docker ____ web
```
Show answer
```
docker port web
```
Match.

container port

What the app listens on
ana makine portu

Reached from outside
EXPOSE

Documentation
0.0.0.0

All interfaces

Next lesson: The -p flag

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies