I know that in an Ethernet LAN, the ARP protocol is used to get a MAC address from an IP address. How does my router get the MAC address of another router on an external network if ARP is not possible?
Mac – How does a router find the MAC address of next hop router
arpmac addressnetworking
Related Solutions
Unless you have something like Virtualbox installed (which installs its own set of virtual LAN cards), you can try typing getmac at the command prompt. It should provide you your LAN card's MAC address.
If you do have Virtualbox installed, you might consider running getmac /v instead for more verbose output. It will list down the connection name associated with the MAC address.
When a packet crosses a subnet boundary (e.g. by reaching a LAN's router), the MAC address is discarded because it means absolutely nothing outside of the subnet in which it originated. It is not possible to recover the MAC address of somebody else's machine if your only method of getting to them involves a router. (The original MAC address is not transmitted to the next hop.) The Layer 3 data, which is the payload of the Layer 2 packet, needs to be repackaged.
When the router needs to send the packet to the next router in the chain, what happens depends on the Layer 2 protocol. If Ethernet is still being used, there will be some MAC address in the sent packets. (You could build routers that communicate with each other over serial ports.) It is theoretically possible to avoid using ARP even with Ethernet, since routing tables could map destination IP addresses to physical ports, removing the switch-like functionality of mapping MAC addresses to those ports. Nevertheless, there is still a field for a MAC address in all Ethernet packets.
Best Answer
Why are you assuming ARP is not possible? ARP is the way its done. There is nothing special about your "internal network" vs an "external network", the same protocols are used on both.
Your router has a set of routes setup to route traffic to your internal network, but it also has a Default Gateway which points to a router in your ISP's network. Your router will use ARP to lookup the MAC address of that default gateway. Then that router will do the same to find its "next hop".