Tapping an Ethernet link
The Plunder Bug by Hak5 is pocket-sized LAN Tap that lets you "bug" Ethernet connections with USB-C convenience.
Using a USB-C cable, connect the Plunder Bug to a computer running a network analyzer such as Wireshark. The light on the Plunder Bug will illuminate green to indicate that it is powered, and you will notice on the computer a new network interface (ASIX AX88772C USB Ethernet).
You will most likely wish to switch the Plunder Bug into passive mode, which mutes the tap port, preventing the host computer from transmitting anything downstream on the tap port. See instructions for your operating system here.