"it depends, nothing good" Network devices forward (switch, more technically) packets to and end device based on an internal MAC table (send packets for DE:AD:

"it depends, nothing good" Network devices forward (switch, more technically) packets to and end device based on an internal MAC table (send packets for DE:AD:BE:EF to interface ge-0/0/0.0) and most devices populate their MAC table simply by looking at input packets and sending the "next" packet for that MAC address out the "last" received interface. If two devices in a network have the same MAC address, they will effectively "fight" for control of the packet flow. You can win that fight by sending a lot of packets. In practice, the other person is going to get annoyed and give up. There are lots of technology which avoid this issue now, but the two primary ones are 802.1x (used in corporate/government environments) and DHCP snooping which can be much more broadly deployed. 802.1x is very complicated and I won't go into it, but, DHCP snooping works by limiting L2 forwarding (MAC table population) to only what the DHCP server says the end device should have and it does this just by inspecting the DHCP replies (no custom protocol) with some vendor specific extensions on the DHCP server side for complex scenarios (you can even do things like put ports in a specific VLAN based on the DHCP reply). This works fine on a physical layer and most hotels are probably using something similar now (less for malicious abusive reasons, though that's a thing) but also just to work around poorly behaving devices and to reduce customer complaints. If you care (and have a modest amount of money) MAC and IP spoofing are dead on the physical layer. For the wifi layer, very similar stuff exists in high-end gear (Rukus/Cisco) and is starting to trickle down to prosumer level gear like unifi. If you care (and have serious cash for Rukus) MAC and IP spoofing are also dead on the wifi layer.