Mac OSX Yosemite on Mac Mini, self assigned ip address

I have had a situation where I caont access the inteet and the network preferences list etheet interface coected to DSL modem has self assigned ip address and caot coect. It is set to coect via dchp

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 coected but I still caot get to remote sites.

One probable cause of the (dchp) self assigned ip address issue was a system up grade or a crash or other significant event. For this there has been nothing like that.

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

Any thought, suggestions, info Thanks for time and attention

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