Computer crackers can gain unauthorized access to the ISP via the same technique.
Moreover, the client is not the only person who can spoof their MAC address to gain access to the ISP. While this is generally a legitimate case, MAC spoofing of new devices can be considered illegal if the ISP's user agreement prevents the user from connecting more than one device to their service. In this case, the client spoofs their MAC address to gain Internet access from multiple devices. This can be circumvented easily by MAC spoofing, with the client only needing to spoof the new device's MAC address so it appears to be the MAC address that was registered by the ISP. Since MAC addresses are unique and hard-coded on network interface controller (NIC) cards, when the client wants to connect a new device or change an existing one, the ISP will detect different MAC addresses and might not grant Internet access to those new devices. Many ISPs register the client's MAC address for service and billing services. New hardware for existing Internet Service Providers (ISP) MAC spoofing is done for legitimate and illicit purposes alike.
