What are the three IPv6 address types, how is a packet sent to each one delivered, and why is there no broadcast?

## Three types, sorted by who receives the packet RFC 4291, the IPv6 addressing architecture, defines exactly three address types and classifies them by **delivery**: how many interfaces receive a packet sent to the address. | Type | Who receives the packet | How you recognise it | Examples | |---|---|---|---| | **Unicast** | the one interface the address identifies | any address outside `ff00::/8` (other than `::`) | `2001:db8:0:7::10`, `fe80::1c4b:72e9:a05d:3f18`, `::1` | | **Multicast** | every interface that has joined the group | the first byte is `ff` | `ff02::1`, `ff02::2`, `ff02::1:ff6a:31b7` | | **Anycast** | one interface of a set, the nearest by the routing protocols' measure of distance | it cannot be recognised: it uses unicast formats | the Subnet-Router anycast address `2001:db8:0:7::` of the link `2001:db8:0:7::/64` | Addresses are assigned to **interfaces**, not nodes, and one interface normally holds several addresses of different types at once. ## Unicast: one interface, several reaches - **Global unicast**: routable across the internet; the everyday address of a server or a laptop. - **Link-local** (`fe80::/10`): every interface is *required* to have one, even when a global address is also present. It is valid only on its own link, and RFC 4291 says routers must not forward any packet with a link-local source or destination to another link. - **Loopback** (`::1`): used by a node to send to itself. It must not be assigned to any physical interface, it is treated as link-local scope, it must never be sent outside the node, and a packet arriving on an interface with destination `::1` must be dropped. - **Unspecified** (`::`): not the address of anything; it marks the absence of one. A node that has no address yet may use it as a source, it must never be a destination, and a router never forwards a packet whose source is `::`. How a prefix such as `fe80::/10` is written and compressed is a notation question; what matters here is each address's delivery role. ## Multicast: one packet, a group of receivers A multicast address names a **group**. An interface may belong to any number of groups, and it joins one with Multicast Listener Discovery (MLD; version 2 is RFC 3810), so routers, and switches that snoop MLD (RFC 4541), know where the members are. - A multicast address **must not** be used as a packet's source. - On Ethernet, RFC 2464 maps a group to a MAC address of `33:33` followed by the group's last four octets, so a network card can discard frames for groups its host has not joined. - Some groups are predefined: **all-nodes** `ff02::1`, **all-routers** `ff02::2`, and one **solicited-node** group for each address a node holds. ## Why there is no broadcast RFC 4291 states it in one line: there are no broadcast addresses in IPv6, "their function being superseded by multicast addresses." The reasoning: 1. An IPv4 broadcast, such as an ARP request, is delivered to every host on the segment, and every host's network stack must examine it, even though only one host cares. 2. IPv6 sorts the same traffic into **narrower groups**. Address resolution goes to a solicited-node group that, in practice, only the target has joined. A Router Solicitation goes to all-routers, a group hosts do not join. 3. When every node genuinely must hear something, such as an unsolicited Router Advertisement, IPv6 still has `ff02::1`. Every node listens to it permanently, without sending MLD reports, and a snooping switch forwards it on all ports. So "no broadcast" does not mean "no way to reach everyone": reaching everyone is one named group among many, used only where it is needed. On a switch that does not snoop MLD, multicast frames are flooded much as broadcasts are; the saving then comes from each receiving card filtering on the `33:33` MAC address instead of passing every frame up to its IP stack. ## Anycast in brief An anycast address is a unicast-format address assigned to more than one interface. The holders must be explicitly configured to know it is anycast, Duplicate Address Detection is never run on it (RFC 4862), and routing delivers each packet to one holder. RFC 4291 predefines one: the **Subnet-Router anycast** address, the subnet prefix with an all-zero interface identifier, which every router must support for the subnets it is attached to. ## Common confusions - "`ff02::1` is IPv6's broadcast address": it plays broadcast's old role, but it is an ordinary multicast group, scoped to one link. - "Anycast has its own prefix": it does not, which is precisely why a sender cannot tell an anycast address from a unicast one. - "Link-local is a fallback for when nothing else works": it is mandatory and sits beside the global addresses all the time.