[BobJones19]

Updated to Yosemite and have no network connections on a iMac (27-inch, Late 2009)

[kch50428]

Shut down the mac... then, if you have control over the network devices(s) your mac connects to - restart them, then fire up the mac.

[BobJones19]

that did not work what did was using the network assistant to to diagnostic the wired network connection it found nothing wrong with it but fixed both wired and wifi, checking the wifi connection did nothing by the way.

[Daniel Dostie]

Same problem for me (Late 2009 27ìnches iMac), Network is OK for 1 min or 20 after reboot and go down. With Safe mode StartUp, it can last 2 hours but everything is slow.