The first 24 bits (0000.0c) are the vendor ID of the address; in the case of HSRP being
a Cisco protocol, the ID is assigned to Cisco.
uu The next 16 bits (07.ac) are the well-known HSRP ID. This part of the address was
assigned by Cisco in the protocol, so it’s always easy to recognize that this address is
for use with HSRP.
uu The last 8 bits (0a) are the only variable bits and represent the HSRP group number
that you assign. In this case, the group number is 10 and converted to hexadecimal
when placed in the MAC address, where it becomes the 0a that you see.