When you need this WAN port MAC address, you can refer the MAC label in the enclosure. The larger one is for WAN port. But most easy way is to use Web-based GUI to check it. Please enter Status -> Device Information or WAN -> ISP -> Obtain an IP address automatically, then you will see the MAC address for WAN port. Usually, some cable operators need this information for registration. This function is available in Application Firmware Version 1.17 and later.