The capability of the cabling after it has been installed, poor installation practice can dramatically affect the performance of data cables.
Tight bends, kinks, overtightened cable ties, crushing, poor termination can all reduce the performance of the cable but still pass a basic continuity test.
A 'proper tester' doesn't confirm continuity or just have flashy lights. You are thinking of the basic continuity checkers available for ~ÂŁ5, a 'proper' tester is more likely to set you back ÂŁ5000.
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Not by a simple Y split, you need a network switch to 'split' a network connection.
Network connections carry a digital data signal, if you attempt to y split the cable you will have all sorts of problems. Imagine trying to listen to two different pieces of music through the same speaker at the same time, that a basic idea of the what the device at the other end of the network connection will be facing.