OS X El Capitan: Set advanced Ethernet options
In most cases, your Ethernet settings are configured automatically. In some network environments, you may need to set advanced options. Your network administrator or ISP should be able to tell you whether you need to adjust these settings.
Unless your network administrator has given you specific settings, use the automatic settings. Entering the wrong settings can affect network performance.
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Choose Apple menu > System Preferences, then click Network.
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Select Ethernet in the list at the left.
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Click Advanced, click Hardware, click the Configure pop-up menu, then choose Manually.
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Click the Speed pop-up menu, then choose a speed.
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Click the Duplex pop-up menu, then choose an option.
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Click the MTU pop-up menu, then choose the standard packet size (1500) or a custom size.
Check with your network administrator or ISP for the settings specific to the network.
If your computer and the device you’re connecting to support Gigabit Ethernet but the Ethernet cable you are using does not, you may need to adjust the Ethernet speed. Alternatively, you can purchase a cable that supports Gigabit Ethernet.