IPv6 is a technology developed to overcome the limitations of the current standard, IPv4, which allows end systems to communicate and forms the foundation of the Internet as we know it today. This section on IPv6-specific design considerations provides an overview of IPv6 features and addressing and explains the various IPv6 address types. The address assignment and name

IPv6 Features

The ability to scale networks for future demands requires a limitless supply of IP addresses and improved mobility; IPv6 combines expanded addressing with a more efficient and feature-rich header to meet these demands. IPv6 satisfies the increasingly complex requirements of hierarchical addressing that IPv4 does not support. The Cisco IOS supports IPv6 in Release 12.2(2)T and later. The main benefits of IPv6 include the following:

■ Larger address space: IPv6 addresses are 128 bits, compared to IPv4’s 32 bits. This larger addressing space allows more support for addressing hierarchy levels, a much greater number of addressable nodes, and simpler autoconfiguration of addresses.

■ Globally unique IP addresses: Every node can have a unique global IPv6 address, which eliminates the need for NAT.

■ Site multihoming: IPv6 allows hosts to have multiple IPv6 addresses and allows networks to have multiple IPv6 prefixes. Consequently, sites can have connections to multiple ISPs without breaking the global routing table.

■ Header format efficiency: A simplified header with a fixed header size makes processing more efficient.

■ Improved privacy and security: IPsec is the IETF standard for IP network security, available for both IPv4 and IPv6. Although the functions are essentially identical in both environments, IPsec is mandatory in IPv6. IPv6 also has optional security headers.

■ Flow labeling capability: A new capability enables the labeling of packets belonging to particular traffic flows for which the sender requests special handling, such as nondefault quality of service (QoS) or real-time service.

■ Increased mobility and multicast capabilities: Mobile IPv6 allows an IPv6 node to change its location on an IPv6 network and still maintain its existing connections. With Mobile IPv6, the mobile node is always reachable through one permanent address. A connection is established with a specific permanent address assigned to the mobile node, and the node remains connected no matter how many times it changes locations and addresses.

IPv6 Address Format

Rather than using dotted-decimal format, IPv6 addresses are written as hexadecimal numbers with colons between each set of four hexadecimal digits (which is 16 bits); we like to call this the “coloned hex” format. The format is x:x:x:x:x:x:x:x, where x is a 16-bit hexadecimal field. A sample address is as follows: 2035:0001:2BC5:0000:0000:087C:0000:000A

IPv6 Packet Header

The IPv6 header has 40 octets, in contrast to the 20 octets in the IPv4 header. IPv6 has fewer fields, and the header is 64-bit-aligned to enable fast, efficient, hardware-based processing. The IPv6 address fields are four times larger than in IPv4. IPv6 contains fields similar to 7 of the 12 IPv4 basic header fields (5 plus the source and destination address fields) but does not require the other fields. The IPv6 header contains the following fields:

■ Version: A 4-bit field, the same as in IPv4. For IPv6, this field contains the number 6; for IPv4, this field contains the number 4.

■ Traffic class: An 8-bit field similar to the type of service (ToS) field in IPv4. This field tags the packet with a traffic class that it uses in differentiated services (DiffServ) QoS. These functions are the same for IPv6 and IPv4.

■ Flow label: This 20-bit field is new in IPv6. It can be used by the source of the packet to tag the packet as being part of a specific flow, allowing multilayer switches and routers to handle traffic on a per-flow basis rather than per-packet, for faster packet-switching performance. This field can also be used to provide QoS.

■ Payload length: This 16-bit field is similar to the IPv4 total length field.

■ Next header: The value of this 8-bit field determines the type of information that follows the basic IPv6 header. It can be transport-layer information, such as Transmission Control Protocol (TCP) or User Datagram Protocol (UDP), or it can be an extension header. The next header field is similar to the protocol field of IPv4.

■ Hop limit: This 8-bit field specifies the maximum number of hops that an IPv6 packet can traverse. Similar to the time to live (TTL) field in IPv4, each router decreases this field by 1. Because there is no checksum in the IPv6 header, an IPv6 router can decrease the field without recomputing the checksum; in IPv4 routers, the recomputation costs processing time. If this field ever reaches 0, a message is sent back to the source of the packet, and the packet is discarded.

■ Source address: This field has 16 octets (128 bits). It identifies the source of the packet.

■ Destination address: This field has 16 octets (128 bits). It identifies the destination of the packet.

■ Extension headers: The extension headers, if any, and the data portion of the packet follow the other eight fields. The number of extension headers is not fixed, so the total length of the extension header chain is variable.

IPv6 Address Types

This section covers the various IPv6 address types and their scopes.

IPv6 Address Scope Types

Similar to IPv4, a single source can address datagrams to either one or many destinations at the same time in IPv6.

Following are the types of IPv6 addresses:

■ Unicast (one-to-one): Similar to an IPv4 unicast address, an IPv6 unicast address is for a single source to send data to a single destination. A packet sent to a unicast IPv6 address goes to the interface identified by that address. The IPv6 unicast address space encompasses the entire IPv6 address range, with the exception of the FF00::/8 range (addresses starting with binary 1111 1111), which is used for multicast addresses. The “IPv6 Unicast Addresses” section discusses the different types of IPv6 unicast addresses.

■ Anycast (one-to-nearest): An IPv6 anycast address is a new type of address that is assigned to a set of interfaces on different devices; an anycast address identifies multiple interfaces. A packet that is sent to an anycast address goes to the closest interface (as determined by the routing protocol being used) identified by the anycast address. Therefore, all nodes with the same anycast address should provide uniform service. Anycast addresses are syntactically indistinguishable from global unicast addresses because anycast addresses are allocated from the global unicast address space. Nodes to which the anycast address is assigned must be explicitly configured to recognize the anycast address. Anycast addresses must not be used as the source address of an IPv6 packet. Examples of when anycast addresses could be used are load balancing, content delivery services, and service location. For example, an anycast address could be assigned to a set of replicated FTP servers. A user in China who wants to retrieve a file would be directed to the Chinese server, whereas a user in the Europe would be directed to the European server.

■ Multicast (one-to-many): Similar to IPv4 multicast, an IPv6 multicast address identifies a set of interfaces (in a given scope), typically on different devices. A packet sent to a multicast address is delivered to all interfaces identified by the multicast address (in a given scope). IPv6 multicast addresses have a 4-bit scope identifier (ID) to specify how far the multicast packet may travel.

IPv6 Name Resolution

This section discusses IPv6 name resolution strategies and name resolution on a dual-stack (IPv4 and IPv6) host.

Static and Dynamic IPv6 Name Resolution

IPv6 and IPv4 name resolutions are similar. The following two name resolutions are available with IPv6:

■ Static name resolution: Accomplished by manual entries in the host’s local configuration files.

■ Dynamic name resolution: Accomplished using a DNS server that supports IPv6, usually along with IPv4 support. As shown in Figure 6-22, an IPv6-aware application requests the destination hostname’s IPv6 address from the DNS server using a request for an A6 record; an A6 record is a new DNS feature that contains an address record for an IPv6 host. The task of querying for the address is done with the name resolver, which is usually part of the operating system. The network administrator must set up the appropriate DNS server with IPv6 support and connect it to the IPv6 network with a valid IPv6 address. The hosts must also have IPv6 addresses.

IPv6 Routing Protocols

The routing protocols available in IPv6 include interior gateway protocols (IGP) for use within an autonomous system and exterior gateway protocols (EGP) for use between autonomous system

■ IGPs: - RIP new generation (RIPng) - EIGRP for IPv6 - OSPF version 3 (OSPFv3) - Integrated IS-IS version 6 (IS-ISv6)

■ EGP: Multiprotocol extensions to BGP version 4 (BGP4+) RIPng RIPng is a distance-vector protocol with a limit of 15 hops that uses split-horizon and poison reverse to prevent routing loops. RIPng features include the following:

■ RIPng is based on the IPv4 RIPv2 and is similar to RIPv2.

■ RIPng uses an IPv6 prefix and a next-hop IPv6 address

■ RIPng uses the multicast address FF02::9, the all-RIP-routers multicast address, as the destination address for RIP updates.

■ RIPng uses IPv6 for transport.

■ RIPng uses link-local addresses as source addresses.

■ RIPng updates are sent on UDP port 521.

EIGRP for IPv6

EIGRP for IPv6 is available in Cisco IOS Release 12.4(6)T and later. EIGRP for IPv4 and EIGRP for IPv6 are configured and managed separately; however, the configuration and operation of EIGRP for IPv4 and IPv6 is similar. EIGRP for IPv6 features include the following:

■ EIGRP for IPv6 is configured directly on the interfaces over which it runs.

■ EIGRP for IPv6 can be configured without the use of a global IPv6 address.