Mac laptop sleeps / WiFi turns off when lid is closed

Mac laptop sleeps when lid is closed

Cause:
OS X enforces system sleep and turns off WiFi when the lid of your Mac laptop is closed *UNLESS* very specific requirements are met.
See https://support.apple.com/en-us/HT201834

While this behavior can be overridden, OS X must be modified with a kernel extension to do so. Apps from the Mac App Store, such as Amphetamine, are not able to modify OS X using a kernel extension.

Resolution:
1) Use an alternative application: https://code.google.com/p/macosx-nosleep-extension/
2) Meet Apple's requirements for using clamshell mode: https://support.apple.com/en-us/HT201834

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