What is the MAC address of the source in the first frame?

To determine the MAC address of the source in the first frame, you would need to analyze the frame's header information, typically found in a network packet capture tool like Wireshark. The source MAC address is usually listed alongside the destination MAC address in the Ethernet frame header. If you have the specific frame data available, you can extract the source MAC address directly from there. If you provide more context or data, I can assist further!