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Our clocks mount flush to the wall using two keyhole slots. Your powered Ethernet cable plugs into a recess in the rear of the clock. Typically, you'd position the open space over the point where your Ethernet cable exits the wall.
However, if you want to install a clock on a solid concrete wall and run an Ethernet cable down the wall to it, probably in a conduit, you might want a drilled 2cm hole and rubber grommet in the top of the case. 2cm is just wide enough to push an Ethernet cable and RJ45 through. We're happy to help!