By the Numbers

By Edoardo Biagioni  |  Posted 2008-02-20 Print this article Print

IPv6 Addresses

2001:503:c27::2:30 and 2001:500:2f::f are two of the root DNS servers for IPv6 domain name service. What do these numbers mean?

IPv4 addresses are 32 bits written in dotted-decimal location, for example Each IPv6 address is 128 bits and is written in hex, four hex digits at a time, and separated by colons. 

For example, 0123:4567:89AB:CDEF:0123:4567:89AB:CDEF as eight groups of four hex digits, separated by colons. Each group of four hex digits represents 16 bits, and the eight groups above represent the 128 bits of a full IPv6 address. Two strategies used to make these numbers more readable to humans are as follows:

+ leading zeros can be omitted from any group of 4 hex digits.

+ a long sequence of zeros can be replaced by :: (at most once per address).

So, using the example above, 2001:503:c27::2:30 really means

Also, a contiguous group of addresses can be written 0123:4567:89AB:CDEF::/64, including all the addresses that have the first 64 bits as shown.

IPv6 addresses really are the reason that IPv6 exists, and the reason that people are considering moving to IPv6.  There are well over 10^28 addresses (written as a "1" followed by 28 "0"s) for each person on the planet, allowing addresses to be used in ways that make routing and address assignment easier and more automatic.

With IPv6 addresses, NAT will become unnecessary (though it can still be an option if people need it), and both mobility and automatic address assignment will be simpler. It will also be easier for organizations to manage their internal networks and assign network numbers to their Internet-connected computers.


A wikipedia page; a good starting point

A site dedicated to furthering the deployment of IPv6; it has a good wiki

A long list of applications in Linux that are IPv6-ready

A good initial link for 6to4

Another good initial link for 6to4

A much-reference IPv6 information page 


Edoardo Biagioni is an Associate Professor in the Department of Information and Computer Sciences at the University of Hawaii at Manoa. He has been teaching networking since 1997, and researching and working in the field of networking since 1991. He has participated in meetings of the IETF and the ATM forum while working for Fore Systems, and worked at Carnegie Mellon University as a Systems Scientist developing new networking protocol implementations. His current research interests include networking protocols, routing, wireless ad-hoc and sensor networks, and network and system security. He can be reached at

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