3.1.b Identify, implement and troubleshoot IPv6 addressing and subnetting

3.1.b [v] Global prefix configuration feature

The upper 64 bits of an IPv6 address are composed from a global routing prefix plus a subnet ID. A general prefix (for example, /48 ) holds a short prefix, based on which a number of longer, more-specific prefixes (for example, /64) can be defined. When the general prefix is changed, all of the more -specific prefixes based on it will change, too. This function greatly simplifies network renumbering and allows for automated prefix definition. For example, a general prefix might be 48 bits long (” /48″) and the more specific prefixes generated from it might be 64 bits long (“/ 64”). In the following example, the leftmost 48 bits of all the specific prefixes will be the same, and they are the same as the general prefix itself. The next 16 bits are all different.

General prefix:

2001: DB8: 2222::/ 48 Specific prefix: 2001: DB8: 2222: 0000::/ 64

Specific prefix:

2001: DB8: 2222: 0001::/ 64 Specific prefix: 2001: DB8: 2222: 4321::/ 64

Specific prefix: 2001: DB8: 2222: 7744::/ 64

General prefixes can be defined in several ways:

● Manually

● Based on a 6to4 interface

● Dynamically, from a prefix received by a DHCP for IPv6 prefix delegation client

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2338-2344).  . Kindle Edition.

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/configuration/15-2mt/ip6-15-2mt-book/ip6-generic-prefix.html#GUID-6D423304-E3A5-4CD3-A884-7932C582E189