Getting a self assigned IP address in Mac OS X 10.10 (Yosemite) on a Mac Mini

Question

I have had a situation where I cannot access the internet and the network preferences list ethernet interface connected to DSL modem has self assigned IP address and cannot connect. It is set to connect via DHCP.

I Googled this problem and the instruction was to remove a file called com.apple.alf.plist from /Library/Preferences and then reboot.

I did this and the interface to the DSL shows that it is connected but I still cannot get to remote sites.

One probable cause of the (DHCP) self assigned IP address issue might have been a system upgrade or a crash or other significant event but there has been nothing like that.

I have been playing with the ethernet cables and now the preferences list the interface as having a self assigned ip address again.

EDIT: This machine has the internet connection to the built in ethernet port and a local network connection via usb/ethernet interface. I switched the internet connection to the USB/Ethernet interface via DHCP and I can now get to the Internet, and I can’t get to other hosts running on local network (now connected via built in ethernet port). I suspect that the built in ethernet port has a hardware problem).

EDIT 7/14:

1. I want local network isolated from internet connected local network
2. I am not looking for this machine to serve as a gateway from one network to another.
3. The built in ethernet port/interface is not working properly
4. I have removed the com.apple.alf.plist file from /Library/Preferences and rebooted with no improvement, accept the "self assigned IP address" status doesn’t show and the Ethernet interface indicates that it is connected, but doesn’t send or receive anything.
5. I ran AHT (Apple Hardware Test) and nothing in the way of hardware was found to malfunction.
6. I have another USB/Ethernet interface and am using that in lieu of using the built in ethernet port/interface. Both USB/ethernet interfaces work fine.

I know how to set interfaces with static private ip addresses, as well as using preferences to specify dchp. My ISP’s DSL modem is expecting interfaces on local hosts to use DCHP.