IPv6 Multicast Addresses (ff00::/8)
Explore IPv6 multicast addresses in the ff00::/8 range. Learn about multicast scopes, well-known addresses like ff02::1, and how IPv6 replaces broadcast with multicast.
Detailed Explanation
IPv6 Multicast: ff00::/8
IPv6 multicast addresses begin with ff and replace the broadcast functionality of IPv4. In IPv6, there is no broadcast — multicast serves that purpose more efficiently.
Address Structure
ff[flags][scope]:group_id
ff = Multicast prefix (8 bits)
flags = 4 bits (0 = permanent, 1 = transient)
scope = 4 bits (scope of the group)
group_id = 112 bits
Multicast Scopes
| Value | Scope | Meaning |
|---|---|---|
| 1 | Interface-Local | Loopback only |
| 2 | Link-Local | Same LAN segment |
| 4 | Admin-Local | Administratively defined |
| 5 | Site-Local | Single site |
| 8 | Organization-Local | Multiple sites in an org |
| e | Global | Internet-wide |
Well-Known Multicast Addresses
| Address | Description |
|---|---|
| ff02::1 | All nodes (link-local) — replaces IPv4 broadcast |
| ff02::2 | All routers (link-local) |
| ff02::5 | OSPFv3 routers |
| ff02::9 | RIPng routers |
| ff02::1:2 | All DHCP agents |
| ff02::fb | mDNS |
| ff02::1:ff00:0/104 | Solicited-node multicast |
Solicited-Node Multicast
IPv6 uses a clever optimization for address resolution (replacing ARP):
Global address: 2001:db8::1234:5678
Solicited-node: ff02::1:ff34:5678
^^^^^^^^^^
Last 24 bits of the address
Instead of broadcasting to all hosts (like ARP), IPv6 sends to a multicast group that only the target host joins, reducing unnecessary processing on other hosts.
Use Case
A network administrator uses ff02::2 to send router advertisements on a link, informing all IPv6 hosts of the network prefix and default gateway without needing to know each host's address.