Multicast addresses
IP multicast addresses
IPv4 multicast addresses:
IANA assigns the Class D address block (224.0.0.0 to 239.255.255.255) to IPv4 multicast.
Table 2: Class D IP address blocks and description
Address block
Description
224.0.0.0 to 224.0.0.255
Reserved permanent group addresses. The IP address 224.0.0.0 is reserved. Other IP addresses can be used by routing protocols and for topology searching, protocol maintenance, and so on. Table 3 lists common permanent group addresses. A packet destined for an address in this block will not be forwarded beyond the local subnet regardless of the TTL value in the IP header.
224.0.1.0 to 238.255.255.255
Globally scoped group addresses. This block includes the following types of designated group addresses:
232.0.0.0/8—SSM group addresses.
233.0.0.0/8—Glop group addresses.
239.0.0.0 to 239.255.255.255
Administratively scoped multicast addresses. These addresses are considered locally unique rather than globally unique. You can reuse them in domains administered by different organizations without causing conflicts. For more information, see RFC 2365.
![[NOTE: ]](images/note.png) | NOTE: "Glop" is a mechanism for assigning multicast addresses between different ASs. By filling an AS number into the middle two bytes of 233.0.0.0, you get 255 multicast addresses for that AS. For more information, see RFC 2770. | |
Table 3: Common permanent multicast group addresses
Address | Description |
|---|---|
224.0.0.1 | All systems on this subnet, including hosts and routers. |
224.0.0.2 | All multicast routers on this subnet. |
224.0.0.3 | Unassigned. |
224.0.0.4 | DVMRP routers. |
224.0.0.5 | OSPF routers. |
224.0.0.6 | OSPF designated routers and backup designated routers. |
224.0.0.7 | Shared Tree (ST) routers. |
224.0.0.8 | ST hosts. |
224.0.0.9 | RIPv2 routers. |
224.0.0.11 | Mobile agents. |
224.0.0.12 | DHCP server/relay agent. |
224.0.0.13 | All Protocol Independent Multicast (PIM) routers. |
224.0.0.14 | RSVP encapsulation. |
224.0.0.15 | All Core-Based Tree (CBT) routers. |
224.0.0.16 | Designated SBM. |
224.0.0.17 | All SBMs. |
224.0.0.18 | VRRP. |
IPv6 multicast addresses:
Figure 4: IPv6 multicast format
The following describes the fields of an IPv6 multicast address:
0xFF—The most significant eight bits are 11111111.
Flags—The Flags field contains four bits.
Figure 5: Flags field format
Table 4: Flags field description
Bit
Description
0
Reserved, set to 0.
R
When set to 0, this address is an IPv6 multicast address without an embedded RP address.
When set to 1, this address is an IPv6 multicast address with an embedded RP address. (The P and T bits must also be set to 1.)
P
When set to 0, this address is an IPv6 multicast address not based on a unicast prefix.
When set to 1, that this address is an IPv6 multicast address based on a unicast prefix. (The T bit must also be set to 1.)
T
When set to 0, this address is an IPv6 multicast address permanently-assigned by IANA.
When set to 1, this address is a transient or dynamically assigned IPv6 multicast address.
Scope—The Scope field contains four bits, which represent the scope of the IPv6 internetwork for which the multicast traffic is intended.
Table 5: Values of the Scope field
Value
Meaning
0, F
Reserved.
1
Interface-local scope.
2
Link-local scope.
3
Subnet-local scope.
4
Admin-local scope.
5
Site-local scope.
6, 7, 9 through D
Unassigned.
8
Organization-local scope.
E
Global scope.
Group ID—The Group ID field contains 112 bits. It uniquely identifies an IPv6 multicast group in the scope that the Scope field defines.
Ethernet multicast MAC addresses
IPv4 multicast MAC addresses:
As defined by IANA, the most significant 24 bits of an IPv4 multicast MAC address are 0x01005E. Bit 25 is 0, and the other 23 bits are the least significant 23 bits of a multicast IPv4 address.
Figure 6: IPv4-to-MAC address mapping
The most significant four bits of an IPv4 multicast address are 1110. In an IPv4-to-MAC address mapping, five bits of the IPv4 multicast address are lost. As a result, 32 IPv4 multicast addresses are mapped to the same IPv4 multicast MAC address. As a result, a device might receive unwanted multicast data at Layer 2 processing, which needs to be filtered by the upper layer.
IPv6 multicast MAC addresses:
As defined by IANA, the most significant 16 bits of an IPv6 multicast MAC address are 0x3333 as its address prefix. The least significant 32 bits are mapped from the least significant 32 bits of an IPv6 multicast address. The problem of duplicate IPv6-to-MAC address mapping also arises like IPv4-to-MAC address mapping.
Figure 7: IPv6-to-MAC address mapping
