1. Make sure your Ethernet cable is plugged in, and verify that you are connected to the network in System Preference > Network.

2. Locate the Network Utility on your mac. It is in Applications > Utilities > Network Utility. Alternatively, search Network Utility in Spotlight Search.

3. In the Info tab, under Network Interface, use the drop-down menu to select the Ethernet.

4. You should be able to view network connection details including network addresses, and connection link speed.

If you have a 100mbps Ethernet Card/Network Interface Card on a Gigabyte connection, upgrade the card to 1Gbps or test speeds with another PC which has a Gigabyte Ethernet Card/Network Interface Card.