I see various instances of ports 2375 and 4243 being used for seemingly the same thing while searching the internet. Also, my local machine requires I use 2375 to connect whereas when I push it to our CI server it requires it be set to 4243.
What does Docker use these ports for and how do they differ?
The docker socket can be configured on any port with the dockerd -H option. Common docker ports that I see include:
Only the first two are set with dockerd -H, swarm mode can be configured as part of docker swarm init --listen-addr or docker swarm join --listen-addr.
I strongly recommend disabling the 2375 port and securing your docker socket. It's trivial to remotely exploit this port to gain full root access without a password from remote. The command to do so is as simple as:
docker -H $your_ip:2375 run -it --rm \
--privileged -v /:/rootfs --net host --pid host busybox
That can be run on any machine with a docker client to give someone a root shell on your host with the full filesystem available under /rootfs, your network visible under ip a, and every process visible under ps -ef.
To setup TLS security on the docker socket, see these instructions. https://docs.docker.com/engine/security/https/