Internet Protocol version 6 (IPv6) Conformanc…

IPv6 extension headers
The extension header is optional in IPv6. If present, extension headers immediately follow the header field. IPv6 extension headers have the following properties:

• They are 64-bit aligned, with much lower overhead than IPv4 options.
• They have no size limit as with IPv4. The only limitation is the size of IPv6 packet.
• They are processed only by destination node. The only exception is the Hop-by- Hop header option.
• The Next Header field of the base IPv6 header identifies the extension header.

Figure 2. IPv4 and IPv6 header formats.

When multiple extension headers are present in a same IPv6 packet, they occur in this order:

• The Hop-by-Hop header carries information that needs to be examined by all the nodes along the delivery path. When present, the Hop-by-Hop option always follows immediately after the basic IPv6 header.
• The Destination header carries additional information that can be examined only by the destination node.
• The Routing header is used by the source node to list all the nodes the packet needs to traverse on the path to its destination.
• The Fragmentation header is used by the source to indicate that the packet has been fragmented to fit within the maximum transmission unit (MTU size). In IPv6, unlike IP4, packet fragmentation and assembly are done by the end nodes instead of routers, which further improves the efficiency of the IPv6 network.
• The Authentication and Encapsulating Security Payload headers (AH and ESP) are used in IPSec to provide security services to ensure the authentication, integrity, and confidentiality of a packet.

IPv6 addressing
The 128-bit IPv6 address is separated into eight 16-bit hexadecimal numbers divided by colons (??. The preferred format is xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx, for example: 2031:0000:1F1F:0000:0000:0100:11A0:ADDF.. The following conventions are also used to represent IPv6 addresses, including ways to shorten them and make them easier to represent:

• Leading zeros can be removed.
• 0000 = 0 (compressed form).
• ?:?represents one or more groups of 16 bits zeros, and can only appear once in an address. For example, 2001:0:13FF:09FF:0:0:0:0001 = 2001:0:13FF:09FF::1
• The lower four 8 bits can use decimal representation of IPv4 addresses. For example, an IPv4-compatible IPv6 address is 0:0:0:0:0:0.192.168.0.1. Unlike an IPv4 node, an IPv6 node allows more than one type of IP address: unicast, anycast, and multicast.

Unicast. An address used to identify a single interface. A packet destined for a unicast address is delivered to the interface identified by that address. Based on the reachability of the packets, unicast supports the following address types.

Global unicast address. An address that can be reached and identified globally. A global unicast address consists of a global routing prefix, a subnet ID, and an interface ID (Figure 3). The current global unicast address allocation uses the range of addresses that start with binary value 001 (2000::/3), one-eighth of the total IPv6 address space.

Site-local unicast address. An address that can only be reached and identified within a customer site, similar to IPv4 private address 10.0.0.0/8 and 192.168.0.0/16. the site-local unicast address contains a FEC0::/10 prefix, subnet ID, and interface ID (Figure 4).

Link-local unicast address. An address that can only be reached and identified by nodes attached to the same local link. A link-local unicast address uses a FE80::/ 10 prefix and an interface ID (Figure 5).

Figure 3. Global unicast address format.

Figure 4. Site-local unicast address format.

Figure 5. Link-local unicast address format.

Anycast. The anycast address is a global address that is assigned to a set of interfaces belonging to different nodes (Figure 6). A packet destined to an anycast address is routed to the nearest interface. The anycast address has the following restrictions:

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