Hello Paul
Indeed you are correct that the 01:00:5E range of MAC addresses is the range that is assigned for use with IPv4 multicast addresses. When you have an IPv4 multicast destination address, when encapsulation takes place, a specific mapping mechanism is used to map to this range. You can see this mechanism here:
The Precision Time Protocol (PTP) however, does not use IP at all! It is a Layer 2 protocol. If you take a look at the packet captures in the lesson, you will see that this is the case. For this reason, a specific multicast address had to be defined that would be used for the purposes of PTP. Since no multicast IPv4 or even IPv6 addresses would be used in this process, the ranges reserved for these cannot be used.
That’s why the IEEE reserved two such MAC group addresses for the purpose of the PTP. They define it like so:
Multicast 802 DA as described in IEEE 1588 Annex F
- Two group addresses are specified, for general messages and for peer-delay mechanism messages, but a Profile may use either address for all messages if it wants to.
- 01-1B-19-00-00-00 – a general group address
- An 802.1Q VLAN Bridge would forward the frame unchanged.
- 01-80-C2-00-00-0E – Individual LAN Scope group address
- An 802.1Q VLAN Bridge would drop the frame
You can find out more information about this definition of these MAC addresses at this IEEE definition (slide 5).
In addition, you can find out more about other ranges of multicast MAC addresses at this NetworkLessons note on the topic.
I hope this has been helpful!
Laz