IP Version 6 Addressing

The short-term solution to the problem of depleting the entire IPv4 address space included NAT and private addressing. CIDR provider a short-term solution to the growing size of IP routing tables. The long-term solution to both problems, at least according to some, is to use a new version of the IP protocol—Version 6 (IPv6). (The version discussed in this book is Version 4. Version 5 was defined for experimental reasons and never was deployed.)

IPv6 calls for a much larger address structure so that the convention of all organizations using unique groupings (networks) of IP addresses still would be reasonable—the numbers of IPv6-style networks would reach into the trillions and beyond. That solution is still technically viable and possibly one day will be used. However, many people believe that the short-term solutions have made IPv4 viable for a long time, and they see no reason to upgrade. Others think that IPv6 is inevitable. Cisco simply supports both options, so whatever happens, it will still be able to sell products.

IPv6 uses a 128-bit IP address written in hexadecimal notation, with colons between each quartet of symbols. How many addresses can you possibly have with a 128-bit number? The only answer that matters in real life is "more than you can possibly ever need." The number is around 3.4 x 1038—yes, that's a 39-digit decimal number. It's huge! You could have a million IP addresses per every person who ever has lived and still not have run out. So, a migration to IPv6 certainly looks like it solves the IP address scalability problem—the only question is whether the other short-term solutions, such as NAT and CIDR, have solved IPv4's problems so well that no one wants to migrate to IPv6.

Table 12-37 summarizes some of the pertinent information comparing IPv4 addresses with IPv6. Table 12-37 IPv4 Versus IPv6




Size of address (bits or bytes per octets)

32 bits, 4 octets

128 bits, 16 octets

Example address


Number of possible addresses, ignoring reserved values


(roughly 4 billion)

2128, or roughly 3.4 * 1038

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