15-3
Router Discovery messages, and ICMPv4 Redirect messages and provides a series of other
functions.
Flexible extension headers
IPv6 cancels the Options field in the header and introduces optional extension headers to
provide scalability and improve efficiency. The Options field in the IPv4 packet header contains
40 bytes at most, whereas the IPv6 extension headers are restricted to the maximum size of
IPv6 packets only.
IPv6 Addresses
IPv6 address format
An IPv6 address is represented as a set of 16-bit hexadecimals separated by colons. An IPv6
address is divided into eight groups, and each 16-bit group is represented by four hexadecimal
numbers, for example, 2001:0000:130F:0000:0000:09C0:876A:130B.
To simplify the representation of IPv6 addresses, zeros in IPv6 addresses can be handled as
follows:
z
The leading zeros in each group can be removed. For example, the above address can be
represented in a shorter format as 2001:0:130F:0:0:9C0:876A:130B.
z
If an IPv6 address contains two or more consecutive groups of zeros, they can be replaced
by a double colon (::). For example, the above address can be represented in the shortest
format as 2001:0:130F::9C0:876A:130B.
A double colon may appear once or not at all in an IPv6 address. Otherwise, the device cannot
determine how many zeros the double colons represent when converting them to zeros to
restore a 128-bit IPv6 address.
An IPv6 address consists of two parts: an address prefix and an interface ID, which are
equivalent to the network ID and the host ID of an IPv4 address respectively.
An IPv6 address prefix is written in the IPv6-address/prefix-length notation where the
IPv6-address is represented in any of the formats above and the prefix-length is a decimal
number indicating how many leftmost bits of the IPv6 address comprise the address prefix.
IPv6 address types
IPv6 addresses fall into three types: unicast address, multicast address, and anycast address.
z
Unicast address: An identifier for a single interface, similar to an IPv4 unicast address. A
packet sent to a unicast address is delivered to the interface identified by that address.
z
Multicast address: An identifier for a set of interfaces (typically belonging to different
nodes), similar to an IPv4 multicast address. A packet sent to a multicast address is
delivered to all interfaces identified by that address.
z
Anycast address: An identifier for a set of interfaces (typically belonging to different nodes).
A packet sent to an anycast address is delivered to the nearest one of the interfaces
