try this
: Remove Network Configuration & Preference Files
Manually trashing the network plist files should be your first line of troubleshooting. This is one of those tricks that consistently resolves even the most stubborn wireless problems on Macs of nearly any OS X version. This is particularly effective for Macs who updated to Yosemite that may have a corrupt or dysfunctional preference file mucking things up:
Turn Off Wi-Fi from the Wireless menu item
From the OS X Finder, hit Command+Shift+G and enter the following path:
/Library/Preferences/SystemConfiguration/
Within this folder locate and select the following files:
com.apple.airport.preferences.plist
com.apple.network.identification.plist
com.apple.wifi.message-tracer.plist
NetworkInterfaces.plist
preferences.plist
Move all of these files into a folder on your Desktop called ‘wifi backups’ or something similar – we’re backing these up just in case you break something but if you regularly backup your Mac you can just delete the files instead since you could restore from Time Machine if need be
Reboot the Mac
Turn ON WI-Fi from the wireless network menu again
This forces OS X to recreate all network configuration files. This alone may resolve your problems, but if you’re continuing to have trouble we recommend following through with the second step which means using some custom network settings.