Actually, it is not. It is a MAC address of some part or counterpart
of the cable head end or ISP-side cable network.
Oddly enough, it differs from the MAC address of the gateway
(or public address of the UBR) only by the last hex digit twosome.
In other words, it looks like this:
I've looked everywhere for a description of DHCPSHADDR, but
cannot find anything definitive.
It's not that important since everything works just fine, but
I hate to leave these little questions unanswered.
A good reference for cable network's and how they
are set up is: