IPv6 Packet Headers and Addressing

IPv6 packet headers contain many of the fields found in IPv4 packet headers; some of these fields have been modified from IPv4. The 40-byte IPv6 header fields are described in Table 10.2.

Table 10.2. IPv6 Packet Header Fields

Field	Description
Traffic class	Class-of-service (CoS) priority of the packet. Previously the type-of-service (ToS) field in IPv4. However, the semantics of this field (for example, DiffServ code points) are identical to IPv4.
Destination address	Final destination node address for the packet.
Flow label	Packet flows requiring a specific CoS. The flow label identifies all packets belonging to a specific flow, and routers can identify these packets and handle them in a similar fashion.
Hop limit	Maximum number of hops allowed. Same as the IPv4 time-to-live (TTL) field.
Next header	Next extension header to examine. Same as the IPv4 protocol field.
Payload length	Length of the IPv6 payload. Same as the IPv4 total length field.
Source address	Address of the source node sending the packet.
Version	Version of the Internet Protocol.

E xtension headers are used to encode optional Internet-layer information. They are placed between the IPv6 header and the upper layer header in a packet. Extension headers are chained together using the next header field in the IPv6 header. The next header field indicates to the router which extension header to expect next. If there are no more extension headers, the next header field indicates the upper layer header (TCP header, UDP header, ICMPv6 header, an encapsulated IP packet, or other items).

IPv6 introduces a new 128-bit addressing model. This creates a much larger address space than IPv4 addresses, which are made up of 32 bits. IPv6 addresses also contain a scope field that categorizes what types of applications are suitable for the address.

IPv6 addresses consist of eight groups of 16-bit hexadecimal values separated by colons (:). The IPv6 address format is as follows :

  aaaa:   aaaa:   aaaa:   aaaa:   aaaa:   aaaa:   aaaa:   aaaa  

aaaa is a 16-bit hexadecimal value, and a is a 4-bit hexadecimal value. Following is an example of an actual IPv6 address:

 3FFE:0000:0000:0001:0200:F8FF:FE75:50DF 

Leading zeros can be omitted, as shown:

 3FFE:0:0:1:200:F8FF:FE75:50DF 

Sixteen-bit groups of zeros can be compressed to "::", as shown here, but only once per address:

 3FFE::1:200:F8FF:FE75:50DF 

There are three types of IPv6 addresses:

• Unicast ”For a single interface.

• Multicast ”For a set of interfaces on the same physical medium. A packet is sent to all the interfaces associated with the address. IPv6 uses multicast addresses to serve the role of IPv4 broadcast addresses.

• Anycast ”For a set of interfaces on different physical medium. A packet is sent to only one of the interfaces associated with this address, not to all the interfaces. There is no equivalent IPv4 address type.

IPv6 addresses have scope, which identifies the application suitable for the address. Unicast and multicast addresses support scoping. Unicast addresses support two types of scope: global scope and local scope. There are two types of local scope: link-local addresses and site-local addresses. Link-local unicast addresses are used within a single network link. The first 10 bits of the prefix identifies the address as a link-local address. Link-local addresses cannot be used outside a network link. Site-local unicast addresses are used within a site or intranet. A site consists of multiple network links, and site-local addresses identify nodes inside the intranet. Site-local addresses cannot be used outside the site. Multicast addresses support 16 different types of scope, including node, link, site, organization, and global scope. A 4-bit field in the prefix identifies the scope.

Unicast addresses identify a single interface. The address consists of n bits for the prefix, and 128 “ n bits for the interface ID.

Multicast addresses identify a set of interfaces. The address consists of the first 8 bits of all ones, a 4-bit flags field, a 4-bit scope field, and 112-bit group ID. The first octet of ones identifies the address as a multicast address. The flags field identifies whether the multicast address is a well-known address or whether it is a transient multicast address. The scope field identifies the scope of the multicast address. The 112-bit group ID identifies the multicast group .

Similar to multicast addresses, anycast addresses identify a set of interfaces. However, packets are sent to only one of the interfaces, not all interfaces. Anycast addresses are allocated from the normal unicast address space and cannot be distinguished from a unicast address in format. Therefore, each member of an anycast group must be configured to recognize certain addresses as anycast addresses.