Three categories of IPv6 addresses exist:
- Unicast – represents a single interface. Packets addressed to a unicast address are delivered to a single host.
- Anycast – identifies one or more interfaces. For example, servers that support the same function can use the same unicast IP address. Packets sent to that IP address are forwarded to the nearest server. Anycast addresses are used for load-balancing. Known as “one-to-nearest” address.
- Multicast – represents a dynamic group of hosts. Packets sent to this address are delivered to many interfaces. Multicast addresses in IPv6 have a similar purpose as their counterparts in IPv4.
IPv6 Unicast Addresses
Unicast addresses represent a single interface. Packets addressed to a unicast address will be delivered to a specific network interface.
There are three types of IPv6 unicast addresses:
- global unicast – similar to IPv4 public IP addresses. These addresses are assigned by the IANA and used on public networks. They have a prefix of 2000::/3, (all the addresses that begin with binary 001).
- unique local – similar to IPv4 private addresses. They are used in private networks and aren’t routable on the Internet. These addresses have a prefix of FD00::/8.
- link local – these addresses are used for sending packets over the local subnet. Routers do not forward packets with this addresses to other subnets. IPv6 requires a link-local address to be assigned to every network interface on which the IPv6 protocol is enabled. These addresses have a prefix of FE80::/10.
IPv6 Global Addresses
IPv6 global addresses are similar to IPv4 public addresses. As the name implies, they are routable on the internet. Currently IANA has assigned only 2000::/3 addresses to the global pool.
A global IPv6 address consists of two parts:
- subnet ID – 64 bits long. Contains the site prefix (obtained from a Regional Internet Registry) and the subnet ID (subnets within the site).
- interface ID – 64 bits long. typically composed of a part of the MAC address of the interface.
Here is a graphical representation of the two parts of an global IPv6 address:
IPv6 Unique Local Addresses
Unique local IPv6 addresses have a similar function as IPv4 private addresses. They are not allocated by an address registry and are not meant to be routed outside their domain. Unique local IPv6 addresses begin with FD00::/8.
A unique local IPv6 address is constructed by appending a randomly generated 40-bit hexadecimal string to the FD00::/8 prefix. The subnet field and interface ID are created the same way as with global IPv6 addresses.
A graphical representation of a unique local IPv6 address:
IPv6 Link-Local Addresses
Link-local IPv6 addresses have a smaller scope of how far they can travel: only within a network segment to which a host is connected. Routers will not forward packets destined to a link-local address to other links. A link-local IPv6 address must be assigned to every network interface on which the IPv6 protocol is enabled. A host can automatically derive its own link-local IP address, or the address can be manually configured.
Link-local addresses have a prefix of FE80::/10. They are mostly used for auto-address configuration and neighbour discovery.
Here is a graphical representation of a link local IPv6 address:
IPv6 Multicast Addresses
Multicast addresses in IPv6 are similar to multicast addresses in IPv4. They are used to communicate with dynamic groupings of hosts, for example all routers on the link (one-to-many distribution).
Here is a graphical representation of the IPv6 multicast packet:
IPv6 multicast addresses start with FF00::/8. After the first 8 bits, there are 4 bits that represent the flag fields that indicate the nature of specific multicast addresses. The next 4 bits indicate the scope of the IPv6 network for which the multicast traffic is intended. Routers use the scope field to determine whether multicast traffic can be forwarded. The remaining 112 bits of the address make up the multicast Group ID.
Some of the possible scope values are:
1 – interface-local
2 – link-local
4 – admin-local
5 – site-local
8 – organization-local
E – global
For example, the addresses that begin with FF02::/16 are multicast addresses intended to stay on the local link.
The following table lists of some of the most common link-local multicast addresses:
IPv6 Address Prefixes
Here is a summary of the most common address prefixes in IPv6:
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