The interface's physical address. For example, for an
IPv6 interface attached to an 802.x link, this object
normally contains a MAC address. Note that in some
cases this address may differ from the address of the
interface's protocol sub-layer. The interface's
media-specific MIB must define the bit and byte
ordering and the format of the value of this object.
For interfaces which do not have such an address
(e.g., a serial line), this object should contain
an octet string of zero length.