Multicast Addresses

From the very beginning multicast support was an integral part of the IPv6 specification. As a consequence, with IPv6, multicasts have finally reached a level of sophistication such that they even make broadcasts obsolete—so IPv6 doesn't support them anymore.

m 11 11 11

x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x, x x x x x x x x x x x x x x x x x x x x x x x xl x x x x x x x x

Multicast Group ID

Scope: 1=interface-local, 2=link-local, .. . , 5=site-local, Flags: 0=permanent, 1=transient x x x x x x x x x xl x x x x x x.

Fig. 3.4. Multicast addresses

IPv6 multicast addresses look like figure 3.4. They start with an ff00::/8 prefix. The following flag nibble, which is either 0 or 1 for now, defines if the address is permanent or transient. A permanent address is officially assigned for a given purpose by the Internet Assigned Numbers Authority (IANA) or similar, while transient addresses are assigned locally.

The fourth nibble, called the scope nibble, defines the scope of the address. Multicasts support far more different scopes than unicasts; available scope nibble values are

1 interface-local

2 link-local

4 admin-local

5 site-local

6 (unnamed)

7 (unnamed)

8 organization-local

c (unnamed) d (unnamed) e global

Only the values 0, 3 and f are reserved and should not be used.

The rest of the address contains the multicast group ID. The entire multicast address identifies a multicast group. Nodes may subscribe to multicast groups to receive the traffic sent to that group.

Of the permanent multicast groups two are particularly useful: The all nodes link-local multicast group ff02::1 and the all routers link-local multicast group ff02::2.

Using these addresses it is possible to ping all nodes ("devices that speak IPv6") or routers within a subnet, respectively. Since they are link-local addresses, we need to specify the interface again as we did with the link-local unicast addresses.

□ Ping the all nodes multicast address (ff02::1) on your subnet. You should receive replies from all IPv6 nodes connected.

□ Now try the all routers multicast address (ff02::2). If you receive a reply, alert your network administrator; either there is a rogue IPv6 enabled router on the same subnet or you are about to wreak havoc in an IPv6 enabled network during the experiments following in the next chapter.

To track down a rogue router continue reading up to section 4.2.1. There you find out how to figure out the Ethernet address of the router.

Multicasts, and especially multicast routing, are surprisingly complex. In chapter 18 we investigate them in far more detail.

Was this article helpful?

0 0

Post a comment