Why do ports need to be specified twice separated by a colon?

Question

A lot of times, I see ports described twice with a colon like in this Docker Compose file from the Docker Networking in Compose page:

version: "3"
services:

  web:
    build: .
    ports:
      - "8000:8000"
  db:
    image: postgres

networks:
  default:
    # Use a custom driver
    driver: custom-driver-1

I've often wondered why the "8000:8000" and not simply "8000"

Then I saw this example, which has the two ports different:

version: "3"
services:
  web:
    build: .
    ports:
      - "8000:8000"
  db:
    image: postgres
    ports:
      - "8001:5432"

Can someone explain what this port representation means?

Answer 1

The first port is host's port and the second is the remote port (i.e: in the container). That expression bounds the remote port to the local port.

In the example you map container's 8080 port to host's 8080 port, but it's perfectly normal to use different ports (e.g: 48080:8080)