I have two Macbook pros, One with OSX 10.5.8, which don't want to upgrade becuase of boatload of expensive software aboard. The other is running Yosemite. I also have a Mac mini running Yosemite. The older OS Macbook pro has had no problems with built in en0 interface and usb-ethernet adapter for the second interface. I have one interface set to connect to the internet with DCHP. The other is set manually with static ip address. I know enough to keep the internet connected network separate from the local network with different subnet mask: 255.255.255.0 for 188.8.131.52- 254. The Yosemite systems are not letting me use this set up. The builtin ethernet port interface will set itself to self assigned IP address for DCHP. And the second interface set with a local static (manual) address will not route. If I use only one interface I can get on to the internet. With a DSL modem, the DCHP will assign a local address, so I can network the machines through that. But now I have a cable connection and DCHP assigns a remote address and sub net mask. So It means that if the Yosemite systems will not do two interfaces, I cannot network the computes locally. What is it with this situation? Anyone have a real clue? EDIT: Route is probably the wrong term (".. address will not route"). It will not send and receive packets on the network with other hosts on the same network.