https://www.bleepingcomputer.com/news/microsoft/microsoft-defender-will-isolate-undiscovered-endpoints-to-block-attacks/

Microsoft is testing a new Defender for Endpoint capability that will block traffic to and from undiscovered endpoints to thwart attackers' lateral network movement attempts.

As the company revealed earlier this week, this is achieved by containing the IP addresses of devices that have yet to be discovered or onboarded to Defender for Endpoint.