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I recently purchased a new USB ethernet adaptor for my MacBook Air (running 10.8.5) direct from the Apple store, but it doesn't work on the ethernet network at work. The old USB ethernet adaptor works fine, and thus it is not a problem with my ethernet card, cable or the network. Any ideas what might be causing this?

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If you needed to register your MAC address for your computer to be authorized on the network at work, then your problem may be that you have inadvertantly registered the MAC address for the original USB-ethernet adaptor, rather than the MAC address of the internal ethernet interface in you computer. The problem is that the new USB adaptor generates a different MAC address than your old one, and is therefore not recognized by your work network.

To fix this problem, you will need to plug in your new USB ethernet adaptor, get its MAC address and request that this MAC address be authorized on your work network.

You can find the MAC address for your USB ethernet adaptor using System Preferences -> Network -> Advanced... -> Hardware. This will give you the MAC address on the en1 ethernet interface going to the adaptor, rather than the en0 interface going to the computer. The en1 MAC address changes when plug in different USB ethernet adaptors, but the en0 does not.

To see how en1 MAC address changes and find the stable en0 MAC address of your ethernet card, open up a terminal window and type:

$ ifconfig

Then plug in your different adpators, and repeat the ifconfig command.

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