Getting an IP's MAC address from behind a router

MAC addresses do not get passed on by routers. You need to look at the router's ARP table.

Edit:

Unless you have some other form of access to the machine, yes, it is impossible. If you can get onto the network it's on (VNC, RDP, SSH), then you can do it, but otherwise no.

If you're router doesn't give you the ARP table on its web interface, you could see if you can telnet in : many home routers have more advanced functions they hide from the web interface.

It cannot be guaranteed that you can see MAC address of machines behind a router. But there are some protocols that may help you in finding MAC address from anywhere in network. For example to find MAC address of a Windows PC on other network when you know its IP address you can use command

nbtstat -A <IP Address>

To do the same thing from Linux machine (with samba installed) use command

nmblookup -R -S -A <IP Address>

If the machine in question is not a windows machine and there is no way to become part of other subnet. Then the only options are seeing MAC address table in router, or if there is common network wide DHCP server then you can see MAC address in DHCP logs. But I guess you have to be administrator for accessing router or DHCP server.

Hence if there are lot of windows machines on your network you can find MAC address for them even when are not in their subnet. But if network has lot of Linux machines then there is no good very common way to find MAC from other networks.

The above technique wont work on Linux samba servers. They will always return MAC address as 00-00-00-00-00-00.

If the IP in question is not on a directly connected subnet of the router, then the MAC won't be in your router's ARP table.

ARP is only visible / relevant for the last hop of a IP connection (layer 2). Everything between the first routing entity and the last routing entity is layer 3, which doesn't use the ARP of the source or destination entities, but rather the ARP of the next directly-connected routing entity.