I installed Mountain Lion yesterday and find that when my 2 year old iMac comes back from sleep it finds the router and connection but fails to allow internet connection. In other words it indicates the machine is connected but I cannot access Mail or use any Internet browser.
I have two PCs in the house that have no problems, so I assume the problem is neither with the modem or the router. There was never a problem when I was running Lion, it began only after installing Mountain Lion, so once again I assume the problem is with the new operating system. My iMac is circa early 2010. My son's iMac (2012) is not demonstrating the same problem.
The most reliable way to establish an internet connection is to reboot my router (NetGear N300). When the computer reconnects after the reboot, it invariably finds the Internet. I have occasionally been successful Turning Wi-Fi off, and then back on, but most times not.
I see that other users are reporting similar difficulties. Even if no one has an answer I hope that this is brought to the attention of Apple Engineers and a fix is quickly forthcoming.
IMac – Internet connection no longer reliable after installing Mountain Lion
imacinternetmacosNetwork
Related Solutions
Try restoring to default settings. This won't erase any data stored on the Time Capsule, it will reset the network configuration.
Open AirPort Utility (Applications > Utilities > AirPort Utility) and right click on your time capsule. Select Restore Default Settings...
Then wait for it to restart, and follow the setup instructions.
I've had this exact same experience, though not every time. Sometimes it means sites that I have visited recently are unavailable but other sites I haven't visited in ages load just fine.
Usually flushing the DNS cache resolves it.
sudo killall -HUP mDNSResponder
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Best Answer
I tried all the tips imaginable. The only solution was to set the channel on the wireless router on channel 11. It seems to me, the Mountain Lion has a problem when the channel the router is in automatic, then every time the channel is changed, the connection of the Wi-Fi card of MacBook drops.