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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)
                     9  IPv6 Addressing

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IPv6 Address Size and Address Space
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IPv6 Address Space Allocation
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IPv6 Address and Address Notation and Prefix Representation
(Page 4 of 4)

IPv6 Mixed Notation

There is also an alternative notation used in some cases, especially for expressing IPv6 addresses that embed IPv4 addresses. For these, it is useful to show the IPv4 portion of the address in the older dotted decimal notation, since that's what we use for IPv4. Since embedding uses the last 32 bits for the IPv4 address, the notation has the first 96 bits in colon hexadecimal notation, and the last 32 bits in dotted decimal. So to take our same example again from above, in mixed notation it would be shown as:

805B:2D9D:DC28::FC57:212.200.31.255

This isn't really a great example of mixed notation, because embedding usually involves long strings of zeroes followed by the IPv4 address. Thus, zero compression comes in very handy here. Instead of seeing something like this:

0:0:0:0:0:0:212.200.31.255

You will typically see just:

::212.200.31.255

At first glance this appears to be an IPv4 address; you have to keep a close eye on those colons in IPv6!

Key Concept: A special mixed notation is defined for IPv6 addresses whose last 32 bits contain an embedded IPv4 address. In this notation, the first 96 bits are displayed in regular colon hexadecimal notation, and the last 32 bits in IPv4-style dotted decimal.


IPv6 Address Prefix Length Representation

Like IPv4 classless addresses, IPv6 addresses are fundamentally divided into a number of network ID bits followed by a number of host ID bits. The network identifier is called the prefix, and the number of bits used is the prefix length. This prefix is represented by following the address by a slash and then putting the prefix length after the slash. This is the same method used for classless IPv4 addressing with CIDR. For example, if the first 48 bits of our example address were the network ID (prefix), then we would express this as 805B:2D9D:DC28::FC57:D4C8:1FFF/48.

Key Concept: In IPv6 the size of an address’s prefix is indicating by the prefix length that follows the address, separated with a slash, just as this is done in IPv4 classless addressing.


As in IPv4, specifiers for whole networks will typically end in long strings of zeroes. These can be replaced by “::” using zero compression. For example, the 48 bit network ID for the example above is 805B:2D9D:DC28:0:0:0:0:0/48, or 805B:2D9D:DC28::/48. You must include the “::” if replacing the trailing zeroes.


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IPv6 Address Size and Address Space
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IPv6 Address Space Allocation
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Version 3.0 - Version Date: September 20, 2005

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