Only one program can listen on an IP:port combo at a given time
For example, 10.1.2.3:80 would be listening on 10.1.2.3 on port 80.
Consider trying to debug different versions of a web server that listens on port 80
Could only run one copy of the application at a time given that it listens only on that port
Containerized services
Wrapping application in a container means that each container can now listen on that port (as the IP:port combo is different, each container has a different IP)
Can have multiple copies of the containers running at once
But how to connect to container's version of application from the host?
Port mapping
The answer is the use of port mapping, or port forwarding / translation
Port mapping takes a connection to a given IP:port and automatically forwards it to another IP:port combo
In this case, we could map an unused port on our host and point it to port 80 on the container(s)
The Docker engine can handle this automatically if we configure it to
Enabling port mapping
To enable port mapping on a given container, we use the docker run command, and the -p flag
-p <host port>:<container port>
-p 5501:80
Can have multiple -p flags for different ports
repl@host:~$ docker run -p 5501:80 nginx
repl@host:~$ docker ps -a
CONTAINER ID IMAGE ... PORTS NAMES
84266724ff47 nginx ... 0.0.0.0:5501->80/tcp, :::5501->80/tcp coiled_elgamal
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