The first step to securing a network is to restrict physical access to the network devices, network access ports, and other points of entry. Network devices need to be kept in locked communications rooms with access tightly controlled.
Physical security can also include the use of motion detectors (sometimes wired to lights or alarms), cameras (or decoy camera), and other measures and deterrents.
As discussed earlier, employees need to be educated about security threats, such as tailgating, so that they are not threat enablers themselves.
But despite the best intentions, we cannot always rely on these measures to keep out bad guys who may be trying to steal assets, plug into a network ...