Docker · Containers in practice
How does port mapping work in Docker?
Short answer
Publishing with -p maps a host port to a container port through the host's NAT rules. EXPOSE only documents intent and does not make the port reachable.
A container has its own network namespace, so a process listening on port 3000 is not reachable from the host until the port is published. The syntax is hostPort:containerPort, and Docker installs forwarding rules to send traffic to the container IP.
docker run -d -p 8080:3000 api:1.0.0
docker run -d -p 127.0.0.1:8080:3000 api:1.0.0
docker port api-1What to watch
- Publishing on 0.0.0.0 exposes the port on all host interfaces
- Bind to 127.0.0.1 when only a local proxy should reach the container
- Port collisions fail at start time, not at build time
- Docker can allocate a random host port when you publish only the container port
In a cluster, publishing host ports is usually replaced by a service or ingress abstraction. Direct host port mapping limits you to one instance per port per node and complicates scheduling.
Follow-ups you should expect
Interviewers use these to check whether the first answer was understanding or memorisation.
- 1What is the difference between EXPOSE and -p?
- 2Why might you bind a published port to 127.0.0.1?
- 3What happens if two containers publish the same host port?
- 4How does port publishing differ in Kubernetes?
Say it in the interview
Publishing a port with -p maps a host port to a container port using the host's forwarding rules. EXPOSE is only documentation, so it does not make anything reachable. I bind to 127.0.0.1 when a local reverse proxy should be the only entry point, and in a cluster I usually avoid host ports and use a service or ingress instead.
Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.
Related questions
Reading is step one
Now say it out loud.
Understanding an answer and delivering it under pressure are different skills. Rehearse this topic in a live Docker mock interview and get scored feedback on depth, clarity and confidence.