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24.3 IPv6 Addresses
IPv6 Addresses Section 24: Introducing Basic IPv6 IPv6 addresses are represented as a series of eight 16bit hexadecimal fields that are separated by colons (:) in the format: x:x:x:x:x:x:x:x, where x is a 16bit hexadecimal field. Here is an example of an IPv6 address: 2001:0DB8:010F:0001:0000:0000:0000:0ACD The A, B, C, D, E, and F in hexadecimal fields are caseinsensitive. There are two ways to shorten the writing of IPv6 addresses: Omit the leading zeros in a field. The leading zeros are optional, so 09C0 can be written as 9C0. A field that contains all zeros (0000) can be written as 0. Use a double colon (::) to represent successive fields of zeros. An address parser can identify the number of missing zeros by separating the two parts and filling in zeros until the 128 bits are completed. However, if two double colons are placed in the address, there is no way to identify the size of each block of zeros. Therefore, only one double colon can be used in an IPv6 address. The figure shows an example of how to shorten an IPv6 address. IPv6 supports three basic types of addresses, which are illustrated in the next figure. Each address type has specific rules regarding its construction and use. Unicast: Unicast addresses are used in a onetoone context. There are several types of IPv6 unicast addresses. Multicast: A multicast address identifies a group of interfaces. Traffic that is sent to a multicast address is sent to multiple destinations at the same time. An interface may belong to any number of multicast groups. Anycast: An IPv6 anycast address is assigned to an interface on more than one node. When a packet is sent to an anycast address, it is routed to the nearest interface that has that address. The nearest interface is found according to the measure of distance of the particular routing protocol. All nodes that share the same address should behave the same way so that the service is offered similarly, regardless of the node that services the request. IPv6 has no support for broadcast addresses in the way that they are used in IPv4. Instead, specific multicast addresses are used. Just as in IPv4, a provision has been made in IPv6 for a special loopback address for testing. Datagrams that are sent to this address “loop back” to the sending device. The loopback address in IPv6 functions in the same way as the loopback address in IPv4 (127.0.0.1). The IPv6 loopback address cannot be assigned to a physical interface. A packet that has the IPv6 loopback address as its source or destination address must remain within the node that created the packet. IPv6 routers do not forward packets that have the IPv6 loopback address as a source or destination address. The IPv6 loopback address is 0:0:0:0:0:0:0:1, which is normally expressed as “::1”. Up Next: IPv6 Address Scopes and Prefixes