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World D1: First Containers

The order hatch: port mapping

You will learn: to open a door from your laptop into a container with -p.

Checked with Docker Engine 29 on 31 Oct 2026

The problem

Your laptop~

  • No open hatches

Lunchboxes (containers)

  • webrunning
    from nginx:1.30-alpine

Frozen packs (images)

  • nginx:1.30-alpine8 layers · 93.5MB · in use

web is Up, but the real error says Couldn't connect to server. Plain words: nobody answers at port 8080 on your laptop. The box is sealed.

Try it

Cut an order hatch: plug laptop port 8080 into web’s port 80. This is port mapping: The order hatch: a door from a port on your computer to a port in the container..

Open the order hatch: laptop port 8080 to web port 80.

Tap a laptop port, then a container port.

Your laptop

web

$ docker run -d --name web nginx:1.30-alpine

Watch

Your laptop~

  • No open hatches

Lunchboxes (containers)

  • webrunning
    from nginx:1.30-alpine

Frozen packs (images)

  • nginx:1.30-alpine8 layers · 93.5MB · in use

A hatch is cut when a box is made. So web is made again. Read -p 8080:80 left to right: laptop first, box second.

Try it

Your turn. Run web with a hatch on 8080. Then curl localhost:8080.

Your laptop~

  • No open hatches

Lunchboxes (containers)

  • No containers yet

Frozen packs (images)

  • alpine:3.221 layers · 12.9MB
  • nginx:1.30-alpine8 layers · 93.5MB

Predict

web holds hatch 8080. A new box, api, also asks for laptop port 8080. What happens?

Check

Your laptop~

Lunchboxes (containers)

  • webrunning
    from nginx:1.30-alpine 8080→80

Frozen packs (images)

  • alpine:3.221 layers · 12.9MB
  • nginx:1.30-alpine8 layers · 93.5MB · in use

The real error ends with port is already allocated. Plain words: another box already uses laptop port 8080.

Try it

Give api its own hatch. Any free laptop port works.

api needs a hatch too. 8080 is busy.

Tap a laptop port, then a container port.

Your laptop

web

api

  • -p 8080:80laptop 8080 → web:80
$ docker run -d --name web -p 8080:80 nginx:1.30-alpine$ docker run -d --name api nginx:1.30-alpine

Check

docker run -d -p 3000:80 nginx:1.30-alpine. Which address opens the page?

You did it!

You can now open an order hatch into a container.

Go deeper (optional)

Without an address, -p opens the hatch on every network you are on. Others on your Wi-Fi may reach it! -p 127.0.0.1:8080:80 keeps it for your laptop only. More hatch tricks come in World D3.

Real rep (optional):

Real mission

  1. docker run -d --name web -p 127.0.0.1:8080:80 nginx:1.30-alpine
  3. docker port web
  4. docker rm -f web

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