Using two ethernet interfaces for separate local networks on Mac OSX Yosemite?

Question

I have a Macbook Pro with only one builtin in ethernet port. I have a separate USB-Ethernet adapter for the second lan network.

I have not been able to get both interfaces to work. Ether the builtin is used, or the USB-ethernet interface works.

I want one local network to connect to DSL modem/router for internet connection. I want the other network to not have internet connection.

The network interface with internet connection is set to DCHP The network interface without internet connection is set to inet 192.168.1.23 subnet 255.255.255.0

Often the built in will auto configure itself using DCHP to a self assigned and unusable internet address. I have found material on how to solve that but the solution is bogus. While it will make the configuration appear to be correct, there is no internet connection, nor connection to other hosts on that local lan.

If the other interface is connected and configured, no connections will be made through it to hosts on that lan either.

I have done this successfully on earlier 10.5 and 10.6 systems, and as well with Linux and FreeBSD systems.

I am confused about what bridging really means and this sounds like the system will not recognize multiple routes.

Any info and suggestions, guidance apprecaited: thank you for time and attention JK

Further info: the internet connected interface is set to DCHP and the network address assigned is 192.168.0.5 with a net mask of 255.255.255.0, The router is the DSL modem/router at 168.162.0.1. The net mask for each sub net should keep them isolated from one-another.

I appreciate the response, but the problem is deeper that just setting the interface priority, I believe: When network preferences shows that the interfaces are configured properly, the output from console: ifconfig only shows EITHER one (en0), or the other (en1) is actually configured with a network address and sub net mask. At the time being, I have the builtin turned off and have unplugged. The other (USB-ethernet) is configured to connect to internet connected LAN. It is work and "if its working, don't fix it" (not knowing exactly how to improve the situation).

Answer 1

Go to System Preferences > Network, look at the bottom of the list of network interfaces, click the “gear” icon button, and select “Set service order…”. Then, in the sheet that pops up, find the list item for your network interface that has the Internet connection, and drag it higher in the list than the network interface for your dead-end LAN.

macOS automatically tries to use the highest-ranked active interface as its default network connection.

Make sure the two networks are using different IP subsets. Since you're using 192.168.1.0/24 on one network, make sure you’re not using it on the other network.