This is the default network type in Docker. When the Docker service daemon starts, it configures a virtual bridge, named docker0. If you do not specify a network with the docker run -net=<NETWORK> option, the Docker daemon will connect the container to the bridge network by default. Also, if you create a new container, it will be connected to the bridge network. For each container that Docker creates, it allocates a virtual Ethernet device which will be attached to the bridge. The virtual Ethernet device is mapped to appear as eth0 in the container, using Linux namespaces, as you can see in the following diagram:
The in-container eth0 ...
