Global Unicast Address - 2000::/3

Global Unicast Address are public routable address.  They are maintained by Internet Corporation for Assigned Names and Numbers (ICANN).  Before you can use this type of address you must (or to some folks, you SHOULD is all we can say) register and obtain a specific address block for your organization's use.  In most cases of small bussinesses these will be assigned to you by your ISP and you need not to register it.

A typical glboal unicast address has a 48 bit global routing prefix followed by 16 bit subnet and 64 bit interface ID as illustrated below.

• 48 bitsGlobal Routing Prefix
• 16 bitsSubnet
• 64 bitsInterface ID

• 2000:0000:0000
• 0000
• 0000:0000:8190:3426

• Link-local
• Site-local
• Global

The 48 bit global routing prefix is further broken up into four groups: prefix, Top Level Aggregator ID (TLA ID), Reserved block and Next Level Aggregator ID (NLA ID).

• 3 bitsPrefix
• 13 bitsTLA ID
• 8 bitsReserved
• 24 bitsNLA ID

The 3-bit prefix of a global unicast address is set to all 1's.  TLA ID is managed by IANA which allocates addresses to local internet registries who then in turn allocate TLA ID to global ISP.  NLA ID is managed by the global ISP by combining the Prefix, TLA ID, Reserved block and NLA ID to form a 48 bit global routing prefix which would be used to identify a major site.  The 8-bit Reserved block is currently reserved for future use to expand the size and scope of either TLA or NLA ID.

Click on this link to examine this global unicast address using the Advanced Online IPv6 Subnet Calculator: 2000:0000:0000:0000:0000:0000:8190:3426