IP Multicasting – Part 2 – Addresses

As I wrote yesterday, the class D range (224.0.0.0 – 239.255.255.255) is reserved for Multicast use.

Those addresses are again split up in several ranges:
224.0.0.0 – 224.0.0.255 (224.0.0.0/24) – Permanent group for local segment: They are not routed
224.0.1.0 – 224.0.1.255 (224.0.1.0/24) – Permanent group for the whole network: They are forwarded in the entire network
232.0.0.0 – 232.255.255.255 (232.0.0.0/8) – Source-Specific Multicast (SSM) applications
233.0.0.0 – 233.255.255.255 (233.0.0.0/8) – GLOP addressing
239.0.0.0 – 239.255.255.255 (239.0.0.0/8) – Private domain multicast
Everythinge else – Transient group

Permanent Groups
There are two ranges of permanent IP multicast addresses: the first is used for packets that are not forwarded by routers, the second when the packets should be forwarded by routers.
An example: The multicast addresses used by OSPF or EIGRP are in the first range, where only routers located on the same segment can communicate to each other and exchange routes.

Source-Specific Multicast (SSM)
Source-Specific multicast allows you to limit the source address to a specific address. This improves security as no other hosts are able to send traffic to the multicast group.

GLOP Addressing
GLOP addressing allocates each registered AS number 256 global multicast IP addresses. As the 2nd and 3rd octet of the IP address, the 16bit ASN is used.
For example if you are using AS 3303:
3303 convered to binary: 00001100 11100111
00001100 is used as the 2nd octet, decimal: 12
11100111 is used as the 3rd octet, decimal: 231
So, the resulting range would be: 239.12.231.0 – 239.12.231.255

Private Multicast Domain
The private multicast domain addresses can be used within the own network, somewhat like the private unicast addresses defined in RFC1918.

Transient Groups
Transient multicast addresses are globally unique multicast addresses. They get dynamically allocated and released when no longer in use. Any IP addresses in the range 224.0.0.0 to 239.255.255.255 that is not assigned to a specific group/range, is considered as a transient multicast address.