/etc/ethers contains 48 bit Ethernet addresses and their corresponding IP numbers, one line for each IP number:
The two items are separated by any number of SPACE and/or TAB characters. A # at the beginning of a line starts a comment which extends to the end of the line. The Ethernet-address is written as x:x:x:x:x:x, where x is a hexadecimal number between 0 and ff which represents one byte of the address, which is in network byte order (big-endian). The IP-number may be a hostname which can be resolved by DNS or a dot separated number.
arp2ethers(8), ether_aton(3), ether-wake(8), rarpd(8), wavemon(1).