Running in an existing network namespace

Normally, Docker creates a new network namespace for each container we run. The network namespace of the container corresponds to the sandbox of the container network model we described earlier on. As we attach the container to a network, we define an endpoint that connects the container network namespace with the actual network. This way, we have one container per network namespace.

Docker provides an additional way to define the network namespace in which a container runs. When creating a new container, we can specify that it should be attached to or maybe we should say included in the network namespace of an existing container. With this technique, we can run multiple containers in a single network namespace:

Multiple containers running in a single network namespace

In the preceding image, we can see that in the leftmost network namespace, we have two containers. The two containers, since they share the same namespace, can communicate on localhost...