📘 CCNA 200-301 v1.1
1.9 Describe IPv6 address types
1.9.c IPv6 Multicast
What is IPv6 Multicast?
In IPv6, Multicast is a method of sending one packet from one source device to multiple destination devices at the same time.
It is used when a device needs to communicate with a group of devices, but not with everyone on the network.
Instead of sending individual copies of the same data to each destination (which wastes bandwidth), the source sends one multicast packet, and the network delivers that packet only to devices that have joined that multicast group.
Key Concept: Multicast Groups
- A multicast group is identified by a special IPv6 address (a multicast address).
- Devices that want to receive multicast traffic “join” a multicast group.
- Devices that are not members of the group ignore the multicast packets.
Example in IT environment:
- Routers may join a multicast group to receive routing updates (like OSPFv3 or RIPng updates).
- Network devices can join a multicast group to receive specific application traffic (like video streaming or service discovery).
IPv6 Multicast Address Format
IPv6 multicast addresses always start with the prefix FF00::/8.
That means every multicast address begins with FF in hexadecimal.
FF00::/8
The format of a multicast address is:
| 8 bits | 4 bits | 4 bits | 112 bits |
|----------|--------|--------|----------------|
| 11111111 | Flags | Scope | Group ID |
Let’s break this down:
| Field | Description |
|---|---|
| 11111111 | Indicates it’s a multicast address (always FF). |
| Flags (4 bits) | Used for special multicast settings (most of the time, this is 0). |
| Scope (4 bits) | Defines how far the multicast traffic can travel in the network. |
| Group ID (112 bits) | Identifies the specific multicast group. |
IPv6 Multicast Scopes
The Scope field is very important — it controls where the multicast traffic is allowed to travel.
| Scope Value | Meaning | Description |
|---|---|---|
| 1 | Interface-local | Multicast packets stay on the same interface (used internally by protocols). |
| 2 | Link-local | Packets stay within the same local link (same network segment). Not forwarded by routers. |
| 5 | Site-local | Packets can travel within the same site (organization’s internal network). |
| 8 | Organization-local | Packets can travel across multiple sites within an organization. |
| E | Global | Packets can travel across the Internet (rarely used). |
In most CCNA-level operations, you’ll mostly deal with link-local (scope 2) multicast addresses.
Common IPv6 Multicast Addresses
Below are important multicast addresses you must know for the CCNA exam:
| Multicast Address | Scope | Purpose |
|---|---|---|
| FF02::1 | Link-local | All nodes on the local link (all IPv6-enabled devices). |
| FF02::2 | Link-local | All routers on the local link. |
| FF02::5 | Link-local | All OSPFv3 routers. |
| FF02::6 | Link-local | All OSPFv3 designated routers (DR/BDR). |
| FF02::9 | Link-local | All RIPng routers. |
| FF02::A | Link-local | All EIGRP routers. |
| FF02::1:2 | Link-local | All DHCP relay agents and servers. |
| FF02::1:FFxx:xxxx | Link-local | Solicited-node multicast address (used in Neighbor Discovery). |
Solicited-Node Multicast Address
One of the most important IPv6 multicast addresses you must understand is the Solicited-Node Multicast Address.
Purpose
Used by IPv6 for Neighbor Discovery Protocol (NDP) — similar to how IPv4 uses ARP to find the MAC address of another host.
Format
Each device automatically joins its own solicited-node multicast address, which is based on the last 24 bits of its IPv6 address.
Example:
If a device has the IPv6 address:
2001:DB8::1A2B:3C4D
The solicited-node multicast address will be:
FF02::1:FF2B:3C4D
So when a device needs to find another host’s MAC address, it sends a Neighbor Solicitation message to this solicited-node multicast address — not to every device on the link.
This helps reduce unnecessary traffic.
Multicast vs Unicast vs Anycast (Quick Comparison)
| Type | Purpose | Example Destination | Delivered To |
|---|---|---|---|
| Unicast | One-to-one communication | 2001:DB8::1 | One specific device |
| Multicast | One-to-many communication | FF02::1 | All members of a multicast group |
| Anycast | One-to-nearest (from many) communication | Assigned unicast-like address | Nearest device in group |
Key Points for the Exam
- All IPv6 multicast addresses start with FF.
- Multicast replaces broadcast in IPv6 (IPv6 does not use broadcast).
- FF02::1 → all IPv6 nodes on local link.
- FF02::2 → all IPv6 routers on local link.
- Solicited-node multicast addresses are used by NDP (like ARP in IPv4).
- Scope field defines how far the multicast message travels.
- Devices must join a multicast group to receive multicast traffic.
- IPv6 routing protocols (like OSPFv3, EIGRP, RIPng) use multicast for exchanging routing updates.
✅ In short for CCNA:
Remember key addresses like FF02::1 (all nodes), FF02::2 (all routers), and FF02::1:FFxx:xxxx (solicited-node).
IPv6 multicast allows efficient one-to-many communication.
No broadcast in IPv6 — multicast replaces it.
