If your Mac's Ethernet connection suddenly drops and you are unable to access the Internet, one of the troubleshooting steps that may resolve the issue is disabling and enabling the Network Interface Card (NIC). This causes Mac OS X to turn the hardware off and on, triggering the NIC to reconnect to the network and obtain a new IP address. Disable and enable the NIC through the "Network" pane of the System Preferences menu.
Click the Apple icon in the upper-left corner of the screen, and select "System Preferences."
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Click the "Network" icon to open the Network Preferences pane.
Click "Ethernet" on the left side of the window.
Click the gear-shaped button next to the minus sign in the lower-left corner of the window, then click "Make Service Inactive."
Click the "Apply" button to disable the NIC.
Click the gear-shaped button again, click "Make Service Active" and click "Apply" to enable the NIC.