|The cat5 cable designed for connecting to a PC or laptop to a router is not the same as cat5 cable designed for connecting a cable modem to a router. The former requires a "straight-through" cable, whereas the latter requires a "crossover" cable.|
The difference is in how the ethernet connectors are wired up at each end of the cable.
See this: http://www.petri.co.il/csc_the_basi...
Actually, many new routers can auto-detect the cable type and configure itself as required regardless of which type of cable is used, but some older routers cannot do this so you have to make sure you use crossover cable from modem to router.