Subnetting IPv6 Addresses

As with IPv4, administrators can create a hierarchy of subnets using IPv6 addresses. However, in IPv6, no subnet masks are needed because there are ample bits in the network identifier to create a subnet identifier without having to borrow from the host bits.

The format for an IPv6 global unicast address divides the 128 bits into the following three sections:
- Global routing prefix A 48-bit field beginning with the 001 FP value, the hierarchical structure of which is left up to the regional Internet registry (RIR)
- Subnet ID A 16-bit field that organizations can use to create an internal hierarchy of subnets
- Interface ID A 64-bit field identifying a specific interface on the network

When you obtain an IPv6 network address from an ISP or an RIR, you typically get the global routing prefix, commonly known as a “/48”. You are then left with the subnet ID field to use for subnetting the network as you wish. Some possible subnetting options are as follows:

- One-level subnet By setting all subnet ID bits to 0, all the computers in the organization are part of a single subnet.
- Two-level subnet By creating a series of 16-bit values, you can split the network into as many as 65,536 subnets. This is the functional equivalent of IPv4 subnetting, but with a much larger subnet address space.
- Multi-level subnet By allocating specific numbers of subnet ID bits, you can create multiple levels of subnets, sub-subnets, and sub-sub-subnets, suitable for an enterprise of almost any size.

For example, consider a large international enterprise with its subnet ID divided as follows:

- Country (4 bits) Creates up to 16 subnets representing the countries in which the organization has offices
- State (6 bits) Creates up to 64 sub-subnets within each country, representing states, provinces, or other geographical divisions
- Office (2 bits) Creates up to four sub-sub-subnets within each state or province, representing offices located in various cities.
- Department (4 bits) Creates up to 16 sub-sub-sub-subnets within each office, representing the various departments or divisions.

Thus, to create a subnet ID for a particular office, you need to assign values for each field. To use the value 1 for the United States, the Country bits of the subnet ID would be as follows:

0001------------
To create a binary state designation for Alaska using the value 49 , the State field would appear as follows:
----110001------

For the second office in Alaska, use the value 2 for the Office bits, as follows:
----------10----
For the Sales department in the office, use the value 9 for the Department bits, as follows:
------------1001
The resulting value for the subnet ID, in binary form, would therefore be as follows:
0001110001101001
In hexadecimal form, that would be 1c69.
Because the organization that owns the prefix wholly controls the subnet ID, enterprise administrators can adjust the number of levels in the hierarchy and the number of bits dedicated to each level as needed.