After the docker installation you have 3 networks by default:
If you start a container by default it will be created inside the bridge (docker0) network.
$ docker run -d jenkins
1498e581cdba jenkins "/bin/tini -- /usr..." 3 minutes ago The –net=host option is used to make the programs inside the Docker container look like they are running on the host itself, from the perspective of the network. It allows the container greater network access than it can normally get.
Normally you have to forward ports from the host machine into a container, but when the containers share the host’s network, any network activity happens directly on the host machine – just as it would if the program was running locally on the host instead of inside a container.
While this does mean you no longer have to expose ports and map them to container ports, it means you have to edit your Dockerfiles to adjust the ports each container listens on, to avoid conflicts as you can’t have two containers operating on the same host port. However, the real reason for this option is for running apps that need network access that is difficult to forward through to a container at the port level.
For example, if you want to run a DHCP server then you need to be able to listen to broadcast traffic on the network, and extract the MAC address from the packet. This information is lost during the port forwarding process, so the only way to run a DHCP server inside Docker is to run the container as –net=host.
Generally speaking, –net=host is only needed when you are running programs with very specific, unusual network needs.
Lastly, from a security perspective, Docker containers can listen on many ports, even though they only advertise (expose) a single port. Normally this is fine as you only forward the single expected port, however if you use –net=host then you’ll get all the container’s ports listening on the host, even those that aren’t listed in the Dockerfile. This means you will need to check the container closely (especially if it’s not yours, e.g. an official one provided by a software project) to make sure you don’t inadvertently expose extra services on the machine.
Reference
https://stackoverflow.com/questions/43316376/what-does-net-host-option-in-docker-command-really-do