IPv6 Part 3 -Multicast Address-
Overview
IPv6 uses the following three types of addresses depending on the communication method.The broadcast address used in IPv4 is obsolete, and IPv6 uses multicast addresses. This article describes unicast addresses.
| Type | Description |
|---|---|
| Unicast Address | An address used to communicate with a specific node. |
| Multicast Addresses | An address used to distribute specific data to multiple nodes. Multiple nodes/interfaces can hold a particular address. |
| Anycast Address | Multiple hosts or interfaces can hold a particular address. An address used to communicate with the nearest node among multiple nodes that hold the same address. |
Previous articles can be found here.
Multicast Address
An address used to communicate with multiple nodes. The first 8 bits begin with FF::/8.
flags
A flags takes the following values depending on the nature of the address.
- 0 ・・・It will always be 0 because it is booked at IANA.
- R ・・・0 (without rendezvous point information), 1 (with rendezvous point information).
- P ・・・0 (without prefix information), 1 (with prefix information).
- T ・・・0 (Wellknown multicast address), 1 (temporary multicast address).
scope
A scope indicates the range of multicast packets to be reached.
| binary | hexadecimal | scope |
|---|---|---|
| 0000 | 0 | Reserved |
| 0001 | 1 | Node-Local scope |
| 0010 | 2 | Link-Local scope |
| 0011 | 3 | Reserved |
| 0100 | 4 | Admin-Local scope |
| 0101 | 5 | Site-Local scope |
| 1000 | 8 | Organization-Local scope |
| 1110 | E | Global scope |
| 1111 | F | Reserved |
Well-known Multicast Address
Like IPv4 addresses, an address pre-defined by IANA for a specific purpose. Basically, the usage is the same as IPv4.
You can check the list at the following URL.
IPv6 Multicast Address Space Registry
An example of Link Local Scope
| Address | Destination | Description |
|---|---|---|
| ff02::1 | All Nodes Adress | Addresses of all nodes to reach all nodes on the same link(RFC4291) |
| ff02::2 | All Routers Address | Addresses of all routers to reach all routers on the same link(RFC4291) |
| ff02::4 | DVMRP Routers | Addresses of all DVMRP (Distance Vector Multicast Routing Protocol) routers used to reach all DVMRP multicast routers on the same link(RFC1075) |
| ff02::5 | OSPFIGP | Addresses of all OSPF (Open Shortest Path First) routers used to reach all OSPF routers on the same link(RFC2328) |
| ff02::6 | OSPFIGP Designated Routers | Addresses of all OSPF-designated routers used to reach all OSPF-designated routers on the same link(RFC2328) |
| ff02::1:ff/104 | Solicited-Node Address | Address of the requesting node used in the address resolution process to resolve the IPv6 address of the link-local node to the link-layer address. The last 24 bits of the requesting node's address are the last 24 bits of the IPv6 unicast address.(RFC4291) |
Solicited-Node Address
Address used to resolve link layer addresses. It is generated based on the following rules.
- Use Network Prefix reserved for Solicited-Node Address.(ff02::1:ff/104)
- Extract the lower 24 bits of the Unicast Address.
- Combine No.1 and No.2 to generate addresses.
MAC address when using Multicast Address
Like IPv4 addresses, when multicast is used for IP addresses, MAC addresses are also generated for multicast. They are generated based on the following rules.
- The first 16 bits of the MAC address are set to [33:33]
- The 32 bits after the MAC address set the lower 32 bits of the multicast address.
- Combine No.1 and No.2 to generate addresses.
