Henry V Child
18c7be1d38
3.9 KiB
| title | date | draft | tags | |||
|---|---|---|---|---|---|---|
| About IPv6 Multicast Addresses | 2025-04-05T18:55:00+02:00 | false |
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In IPv6 multicast is used much more frequently that in legacy IPv4 which is cool because multicast can do some very interesting things such as mDNS and next hop redundancy among others. But how are multicast addresses formed and what do the individual parts of a mulitcast address mean? In this note the elements of a IPv6 Multicast address are explained and different values made clear.
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Prefix
all IPv6 multicast addresses use the address space ff::/8. This means that the first 8 bits of an IPv6 multicast address are always set to 1 or rather are prefixed with ff.
Flags
The next four bits are designated to Flags. The following table shows the 4 flags:
| Flag | Shorthand | Description |
|---|---|---|
| Reserved | 0 | This flag is reserved for future use. It is always set to 0 |
| Rendezvous point | R | This flag is set to 1 when the address for a rendezvous point is embedded in the group identifier. |
| Prefix | P | This flag is set to 1 when a prefix and prefix length in embedded into the group identifier. |
| Transient | T | This bit is set to one when this is it isn't of the well known multicast addresses. |
Scope
The next four bits after the flags belong the the scope. The most important scopes are:
| Scope | Name |
|---|---|
| 1 | Interface-Local |
| 2 | Link-Local |
| 5 | Site-Local |
Group Identifier
The remaining 112 bits are for the group ID.
Well Known Multicast Addresses
| Address | Description |
|---|---|
| ff02::1 | All nodes on the local network segment |
| ff02::2 | All routers on the local network segment |
| ff02::5 | OSPFv3 All SPF routers |
| ff02::6 | OSPFv3 All DR routers |
| ff02::8 | IS-IS for IPv6 routers |
| ff02::9 | Routing Information Protocol routers |
| ff02::a | EIGRP routers |
| ff02::d | Protocol Independent Multicast routers |
| ff02::12 | Virtual Router Redundancy Protocol version 3 |
| ff02::16 | MLDv2 reports |
| ff02::1:2 | All DHCPv6 servers and relay agents on the local network segment |
| ff02::1:3 | All LLMNR hosts on the local network segment |
| ff05::1:3 | All DHCPv6 servers on the local network site |
| ff0x::c | Simple Service Discovery Protocol |
| ff0x::fb | Multicast DNS |
| ff0x::101 | Network Time Protocol |
| ff0x::108 | Network Information Service |
| ff0x::181 | (PTP) version 2 messages (Sync, Announce, etc.) except peer delay measurement |
| ff02::6b | (PTP) version 2 peer delay measurement messages |
| ff0x::114 | Used for experiments |