All iMac computers come built with an internal Ethernet card. The card lets you connect to a local area network (LAN) via an Ethernet cable. But when you're struggling to maintain a solid Internet connection, it can be tempting to assume that the Ethernet card itself is bad. While that is possible, most card issues can be fixed by simply resetting the card to its original configuration. Fortunately, all this can be done in Apple OS X's System Preferences menu.
Turn on your iMac and click on the "System Preferences" icon in your dock menu along the bottom of the screen.
Click on the "Network" icon in the System Preferences menu.
Click on the arrows next to the "Show" option. Select "Network Port Configurations." Click on the check-box next to the "Built-In Ethernet" option. Click on the "Apply" button.
Click on the Apple icon in the upper-left corner of the screen. Select the "Restart" option in the drop-down menu to restart your iMac.
Navigate to the "Network Port Configurations" window through the System Preferences menu. Click on the "Built-In Ethernet" box, and click on the "Apply" button to reset the Ethernet card.