When troubleshooting a network issue, 'Divide and conquer' is a systematic approach used to isolate the scope of the problem. By dividing the network into segments and testing each segment individually, you can identify the specific area that is causing the issue. This differs from the 'Top-to-bottom/bottom-to-top OSI model' approach, which involves checking the layers of the OSI model in sequence either from the top (application layer) downwards or from the bottom (physical layer) upwards, without necessarily first isolating segments of the network. The other options presented are not standard methods for establishing a theory of probable cause.