Get Mac Address Of Devices Connected Via Ethernet On Network

Run arp -a to find out whats connected.

arp -a

Will return.

? (10.18.14.000) at 7c:a:3f:34:c2:2a on en0 ifscope [ethernet]
? (255.255.255.250) at 1:0:5e:7f:yu:fa on en7 ifscope permanent [ethernet]

Use the cmd below to scan.

sudo arp-scan --interface=en0 --localnet

This will give you the mac address of the device connect via ethernet.

If you dont have arp-scan install it.
Mac

brew install arp-scan

Rasperry Pi

sudo apt-get install arp-scan