- Leading zeros in a group can be omitted. The address above can be represented as 2001:8:85a3:0:0:0:11:7334
- Successive fields/groups of zeros can be omitted and replaced by a double colon (::) only single time in an IPv6 address. The address above can be represented as 2001:8:85a3::11:7334
IPv6 networks are written in CIDR notation: a network is denoted by the first address in the network ending in all zeroes, compressed to ::, followed by a slash (/), and a decimal value indicating the size in bits of the address prefix. The IPv6 network for the address above is written as 2001:8:85a3::/64
IPv6 addresses are classified into three types
- Unicast: It identifies a single interface, a packet sent to a unicast address is delivered to the interface which has that specific address configured.
IPv6 unicast addresses are subdivided into the following categories
- Aggregatable global unicast address: The Address structure begins with 3 bits as 001 followed by 45-bit Global routing prefix, 16-bit Subnet ID, and a 64-bit Interface ID. This is an IPv6 address from an aggregatable global unicast prefix, they are used on links that are aggregated upwards through orgazanizations. 2001:DB8:C003:1::F00D is an example of a global unicast address.
- Link Local: These addresses are used by nodes on a local-link to communicate with each other, and IPv6 routers must not forward packets that have link-local source or destination to other links. These addresses are used for various functions such as neighbor discovery and stateless autoconfiguration. Link-Local addresses start with 1111 1110 10 in binary or FE80::/10 in hex. After the first 10 bits in binary there are 54 zeros and then the 64-bit Interface ID. FE80::0060:08FF:FEB1:7EA2
- IPv4-Compatible IPv6 address: This address has zeros in the first 96 bits and an IPv4 address in the last 32 bits. An example of an IPv4-compatible IPv6 address is ::192.168.1.1or ::C0A8:0101. They are assigned to nodes that support both protocol stacks and are used in automatic tunnels.
- Unique Local Address: These addresses are globally unique and are intended for local communications as they are not routable on the Internet instead of in a limited area such as a site. FC00::/7 Prefix identifies Unique Local addresses. Followed by 7-bit prefix is the L-bit, 40-bit global identifier, 16-bit subnet ID and the 64-bit Interface
- Multicast: An IPv6 multicast address is an identifier for a group of interfaces (typically on different nodes). An IPv6 multicast address has a prefix FF00::/8 following is an octet that defines the lifetime and scope of the address.
Shown below is the format of IPv6 Multicast Address
FF02:0:0:0:0:0:0:1 is an example of multicast address. This address is used as the All-nodes multicast group.
- Anycast: An IPv6 unicast address is an address that is assigned to a set of interfaces that typically belong to different nodes. A packet sent to an anycast address will be delivered to the closest interface decided by the routing mechanism. When a unicast address is assigned to more than one interface, it is turned into an anycast address, the nodes to which the address is assigned must be explicitly configured to know that it is an anycast address. Its format is as follows
This brings us to the end of this lesson in which we learned IPv6 address formats and different types of IPv6 addresses. This is one of the key IPv6 lessons as further lessons such as IPv6 Routing and Transition to IPv6 are based on this.