The port concept
- Docker & Kubernetes course
- Beginner
- Lesson 43/82
- 4 min
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 pageSeeing 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 -> 0.0.0.0:80800.0.0.0 means it listens on all of the host's network interfaces: another computer on the same network can reach it too.
Practice 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.
Starter code
# buraya yazSample solution
docker run -d --name web -p 8080:80 nginx
docker port webExpected output
5a6b7c8d9e0f 80/tcp -> 0.0.0.0:8080
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
- Yes, always
Explanation: EXPOSE also tells which ports the -P flag will publish.
2 Show the container's port mappings.
docker ____ webdocker port webExplanation: docker port container lists the mappings.
3 Match.
| Term | Meaning |
|---|---|
container port |
What the app listens on |
ana makine portu |
Reached from outside |
EXPOSE |
Documentation |
0.0.0.0 |
All interfaces |