IP Version 6


IPv6 is the next generation of IP, created to overcome the limitations of IPv4. Although IPv4 has served the Internet well, IPv4 addresses were not allocated efficientlya global shortage of addresses exists, especially in the developing world. The use of private IPv4 addresses and Network Address Translation (NAT) (explained in Chapter 3) has meant that we have been able to cope so far. However, as more people become connected to the Internet with more devices, the ever-increasing need for IP addresses isn't about to disappear.

IPv4 and IPv6 have some similarities and some differences. To compare them, we start with the IPv6 packet header, as illustrated in Figure 10-3.

Figure 10-3. IPv6 Header Includes 128-Bit Source and Destination Addresses


One noticeable difference between the two versions of the Internet Protocol is the size of the address: IPv6 addresses are 128 bits long, four times larger than IPv4 addresses. Those network administrators who struggled with calculating IPv4 subnet masks might wonder how they will cope with 128-bit IPv6 addresses. However, there is good newsthese 128-bit addresses don't have to be typed into devices; rather, IPv6 devices can automatically configure their own addresses (with minimal typing on your part). IPv6 devices can even have multiple addresses per interface.

Other fields of note in the IPv6 header are as follows:

  • Traffic class This 8-bit field is similar to IPv4's type of service (ToS) field, which marks traffic for quality of service (QoS).

  • 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. This feature allows routers to handle traffic on a per-flow basis, rather than per-packet, providing faster processing. The flow label can also be used to provide QoS.

  • Hop limit This 8-bit field is similar to the IPv4 Time to Live (TTL) field. It is decremented by each router that the packet passes through; if it ever reaches 0, a message is sent back to the source of the packet and the packet is discarded.

Rather than using dotted decimal format, IPv6 addresses are written as hex numbers with colons between each set of four hex digits (which is 16 bits); we like to call this the "coloned hex" format. An example address is as follows:

2035:0001:2BC5:0000:0000:087C:0000:000A


Fortunately, you can shorten the written form of IPv6 addresses. Leading 0s within each set of four hex digits can be omitted, and a pair of colons can be used, once within an address, to represent any number of successive 0s. For example, the previous address can be shortened to the following:

2035:1:2BC5::87C:0:A


Similar to how IPv4 subnet masks can be written as a prefix (for example, /24), IPv6 uses prefixes to indicate the number of bits of network or subnet.

The following are the three main types of IPv6 addresses:

  • Unicast Similar to an IPv4 unicast address, an IPv6 unicast address is for a single interface. A packet that is sent to a unicast address goes to the interface identified by that address.

  • Anycast An IPv6 anycast address is assigned to a set of interfaces on different devices. 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.

  • Multicast An IPv6 multicast address identifies a set of interfaces on different devices. A packet sent to a multicast address is delivered to all the interfaces identified by the multicast address.

Broadcast addresses do not exist in IPv6.

There are three main types of unicast addresses,[5] as follows:

  • Global unicast address Similar to IPv4 public unicast addresses, IPv6 global unicast addresses can be used on any network. Addresses in this group are defined by the prefix 2000::/3in other words, the first 3 bits of the hex number 2000, which is binary 001, identify this group of addresses. A global unicast address typically has three fields: a 48-bit global prefix, a 16-bit subnet ID, and a 64-bit interface identifier (ID). The interface ID contains the 48-bit MAC address of the interface, written in an extended universal identifier 64-bit (EUI-64) format.

    EUI-64 Format

    The EUI-64 format interface ID is created by inserting the hex number FFFE between the upper 3 bytes (the Organizational Unique Identifier [OUI] field) and the lower 3 bytes (the serial number) of the MAC address. The seventh bit in the high-order byte is also set to 1 (equivalent to the IEEE G/L bit) to indicate the uniqueness of the 48-bit address.


  • Site-local unicast address These addresses are similar to IPv4 private addresses. They are identified by the FEC0::/10 prefix (binary 1111 1110 11), and they have a 16-bit subnet and a 64-bit interface ID in EUI-64 format.

  • Link-local unicast address This type of address is automatically configured on an interface by using the link-local prefix FE80::/10 (binary 1111 1110 10) and the interface ID in the EUI-64 format. Link-local addresses allow multiple devices on the same link to communicate with no address configuration required.

The IPv6 stateless autoconfiguration process allows IPv6 devices to be automatically configured and renumbered. Routers send out advertisements that include the prefix (/64) to be used on the network. The device then automatically concatenates its MAC address, in EUI-64 format, with this prefix to create its own address.

A few other types of unicast addresses exist; they are used for communicating between IPv4 and IPv6 devices or transporting IPv6 packets over an IPv4 network. These addresses would be used when migrating from IPv4 to IPv6.

Note

Further information on IPv6 can be found at http://www.cisco.com/go/ipv6.





Campus Network Design Fundamentals
Campus Network Design Fundamentals
ISBN: 1587052229
EAN: 2147483647
Year: 2005
Pages: 156

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