IPv6 Well-Known multicast addresses are used in the Group ID portion of the multicast address. They're permanently assigned and can't be changed.

They're registered with IANA so they’re known there. Well-Known addresses are integrated into the OS of many network devices so they’re known there. And soon they’ll be known by you. These addresses really are...“Well-Known”!

Let's take a look at some values in a well-known multicast address.

ff02::1 breaks down as follows:
ff = Multicast
0 = Must always have a 0 here. This bit is reserved for future use
2 = The scope is the local link
::1= The 112-bit group ID (one hundred eleven 0s and one 1). ::1 refers to all nodes.

  • Therefore, ff02::1 is the multicast address for all nodes on the local segment

ff0 = The beginning of every multicast address
2 = The scope for the local link
::2 =The group ID (Well-Know Multicast Address) for all routers

  • Therefore, ff02::2 is the multicast address for all routers on the local segment

Here are some examples with the same Well-Known Multicast Address in the Group ID field, but different scopes:
Ff02::101 = All NTP nodes within the same link
Ff05::101 = All NTP nodes within the same site

Note: Multicast addresses should not be used as a Source address or appear in any routing header.

Below is a list of some common Well-Known multicast addresses from https://www.wikiwand.com/en/Multicast_address.

Well-Known Multicast Addresses
Common Well-Known Multicast Addresses

The Group ID can also be used for a Temporarily Assigned Multicast Address. These addresses work the same as the Well-Known addressing above, except they're assigned by an administrator. They would be useful for a video stream between 10-11AM.

RFC 2375: IPv6 Multicast Address Assignments

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