Use a black fine tip sharpie to mark cables that can be swapped in and out. The id label is easy to fit through the slit as the tags are rubbery and flexible.

The blank ones are a bit pricey. They also come in a 50 pack at about 3x the cost of a 10 pack. It makes it so much safer for others when they have to make changes.

I just call one of my routers r1 and the other r2, so r1 in means exactly what it sounds like. Ditto for the modem (m), etc. Kableflags label power cords well. Used them for under the dash marking of various wire sources on my 62 austin healey.

These are awesome and the best ones out there. They are made of a flexible soft rubber. These work well and appear to be very durable.

Ball point pen ink runs in the description area and looks blurry afterwards. After a week, the ink soaks into the rubber itself leaving an ugly, untidy, smeared word. If you cannot read the label then it's not usable. They just fade on their own.