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What scares most administrators about IPv6 addresses is how different they appear when compared to IPv4 addresses Learning to deal with an address four times longer seems impossible; however, as you will see in this chapter, the standards body for TCP/IP has simplified it as much as possible
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Whereas IPv4 addresses use a dotted-decimal format, where each byte ranges from 0 to 255, IPv6 addresses use eight sets of four hexadecimal addresses (16 bits in each set), separated by a colon (:), like this: xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx (x would be a hexadecimal value) This notation is commonly called string notation Recall from 3 that hexadecimal numbers range from 0 to F Here are some important items concerning IPv6 addresses:
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Hexadecimal values can be displayed in either lower- or upper-case for the
numbers A F
A leading zero in a set of numbers can be omitted; for example, you could
either enter 0012 or 12 in one of the eight fields both are correct
If you have successive fields of zeroes in an IPv6 address, you can represent
them as two colons (::) For example, 0:0:0:0:0:0:0:5 could be represented as ::5; and ABC:567:0:0:8888:9999:1111:0 could be represented as ABC: 567::8888:9999:1111:0 However, you can only do this once in the address: ABC::567::891::00 would be invalid since :: appears more than once in the address The reason for this limitation is that if you had two or more repetitions, you wouldn t know how many sets of zeroes were being omitted from each part
An unspecified address is represented as ::, since it contains all zeroes
24: IPv6
Remember the format of IPv6 addresses: eight sets of 4 hexadecimal numbers If you have successive elds of zeroes in an IPv6 address, you can
represent them as two colons (::) However, you can do this only once in the address: ABC::567::891::00 would be invalid since :: appears more than once in the address
Types of IPv6 Addresses
Recall from 7 that many types of IPv4 addresses exist: unicast, broadcast, multicast, research, private, and so on IPv6 also has different types of addresses Following are the three main types:
Anycast
Very different from an IPv4 broadcast one-to-the-nearest interface, where many interfaces can share the same address Similar to a multicast in IPv4 one to a group of devices Represents a single interface IETF defines support for many data link layer types; however, Cisco supports only Ethernet, Point-to-Point Protocol (PPP), High-Level Data Link Control (HDLC), Asynchronous Transfer Mode (ATM) (using PVCs only), and Frame Relay (using PVCs only) The following sections discuss these in more depth
Multicast Unicast
Be able to compare and contrast anycast, multicast, and unicast IPv6 addresses
Anycast
One of the problems with addressing in IPv4 was the use of broadcasts, which every device had to process on a segment (even when the broadcast wasn t ultimately destined to a device) IPv4 relied heavily on broadcasts to discover devices on a segment, such as ARP, and to acquire addressing, such as Dynamic Host Control Protocol (DHCP) In IPv6, broadcasts no longer exist: they ve been replaced with anycast and multicast addresses An anycast address identifies one or more interfaces Notice that I don t use the term device, since a device can have more than one interface Sometimes people use the term node to designate an interface on a device Basically, an anycast is a
IPv6 Addressing
hybrid of a unicast and multicast address With a unicast, one packet is sent to one destination; with a multicast, one packet is sent to all members of the multicast group; and with an anycast, a packet is sent to any one member of a group of devices that are configured with the anycast address By default, packets sent to an anycast address are forwarded to the closet interface (node), which is based on the routing process employed to get the packet to the destination Given this process, anycast addresses are commonly referred to as one-to-the-nearest address And, interestingly enough, anycast addresses are allocated from the global pool of unicast addresses in IPv6, making a unicast and anycast address indistinguishable from each other when you look at them in a packet And since multiple devices can be configured with the same anycast address, they are commonly used in situations where load balancing traffic, such as web content, to two different destinations is needed Anycast addresses and their uses are still in their infancy and some known problems can occur when using them Until administrators gain more experience in how anycast addresses work and people in the industry agree upon how to solve these problems, you should follow these guidelines: Don t assign these addresses to hosts only routers Don t put an anycast address in the source of a packet only the destination
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