This is one technique for connecting. Mainly there are two kinds of identification
- the ISP provides a router and identifys the customer by the router’s MAC
- the ISP provides a modem which does the identification, the customer is identified by the MAC of the first device connecting to the modem
In case 1. changing the device for WAN access means either to mimic the router by MAC or to register the new MAC at the ISP.
In case 2. a change of the device demands just a restart of the modem.
Case 2. is getting more and more rarely. Most ISP use a combined modem/router which can’t be set to bridge mode ( that is the modem functionality ).