HSRP (Hot Standby Router Protocol) uses virtual MAC addresses for the redundancy group to provide high availability. For HSRP group 15, the default virtual MAC address is 00:00:0c:07:ac:0f. This address is derived from the HSRP group number in hexadecimal format, where "ac" is a fixed prefix and "0f" represents the group number 15 in hexadecimal45. References := Configuring HSRP - Cisco, HSRP Question - Cisco Community