Running Jenkins ver. 1.651.3 on Linux 3.13.0-74-generic #118-Ubuntu. Single installation, no slaves.

I installed the docker-build-step.

Under Jenkins-Configuration-Docker I selected: "Install latest from docker.io"

Under Jenkins-Configuration-Docker-Builder, I followed the Configuration from the plugin page.

Restarted Jenkins.

When I try Test Connection, I only get:
"Something went wrong, cannot connect to http://127.0.0.1:2375/, cause: org.apache.http.conn.HttpHostConnectException: Connect to 127.0.0.1:2375 [/127.0.0.1] failed: Connection refused"

Obviously I am missing something in my configuration, but several hours on Google did not reveal what that is.