My Mac Mini (late 2011) is connected by ethernet to a Topfield PVR. The Topfield has a web server and an FTP server. The connection is direct - there is no router between them as they are only 30cm apart and there are no other devices connected. This arrangement has worked for over a year until now. The ethernet connection has not worked properly since I installed OS X 10.10.5. With 10.10.5, it sometimes took 2-3 attempts to get a connection. I installed the 10.11 GM release. Ethernet to the PVR has not worked since. Tried the 10.11.1 public beta with no change. Installed the public 10.11 release on 1 October but still no ethernet connection visible on the Mini. The green light (power) on the PVR ethernet socket is working and the yellow light (data) blinks. But, the Mac Mini can't see the PVR on the network. I get a "Server is not responding" error in Safari and "file server will not allow additional users" error when trying to connect to ftp. Today, I connected a router between the PVR and the Mac Mini. I set both devices to get their IP addresses from the router instead of static and the connection worked. So, why doesn't the connection work without a router ? What changed in 10.10 and 10.11 that caused the ethernet connection between two devices to require a router ? I've also tried these ideas: Have done an IP scan using a 3rd party utility and found that the PVR is not visible on the network. Checked the cable - unplugged and replugged. Changed the IP addresses of the Mac Mini and PVR (need to be manually set as there is no router). Deleted the Thunderbolt Bridge network service from all locations and rebooted. Tried to assign the same IP address to both devices - silly but, got an error saying the address was being used by another device on the network suggesting that there is some kind of connection. Looked for anything incriminating in the system.log but, can't see anything. I have found a number of other users with similar problems: https://discussions.apple.com/thread/7251574. Does anyone have any ideas ?