With the number of IPv6 network implementations increasing, network engineers will need to learn the ins and outs of IPv6 network subnetting (if they haven’t already). While the general concepts behind the subnetting of IPv6 are not all that different from IPv4 subnetting, it can be a little challenging to get around the size and notation differences between a standard decimal notated IPv4 address and network and the standard hexadecimal notated IPv6 address and network. This article looks at the concepts behind IPv6 subnetting as it is implemented today; keep in mind however that some of the boundaries discussed in this article can change in the future as IPv6 deployment continues.
IPv6 Addressing Types and Assignment Structures
Before going forward that far it is important to note that like IPv4, the placement of the devices that will be allocated with IPv6 addresses can affect the numbers and the method of assignment.
If the IPv6 addresses in question will be used solely for the internal purposes of a business and do not need to be directly routable to the public Internet, then the Unique Local IPv6 range (RFC 4193) and assignment method can be used; these would be the equivalent of RFC 1918 IPv4 private address ranges. If, however, the IPv6 addresses are intended to be directly routable on the public Internet, a range must be assigned (just like with IPv4) by an Internet Service Provider (ISP), or a Regional Internet Registry (RIR) if allocating for an ISP.
As of this writing, the Unique Local address range includes all addresses that fall under the FC00::/7 prefix (they begin with ‘1111110’ in binary), while the Global Unicast address range includes all addresses that fall under the 2000::/3 prefix (they begin with ‘001’ in binary). Global Unicast ranges are divided by a number of different entities; at the top of these entities is Internet Corporation for Assigned Names and Numbers (ICANN) who assigns addresses to the RIR’s (for all regional registries except APNIC), who will in turn assign addresses to ISP’s who will in turn assign addresses to End Users (EU).
The assignment of Unique Local IPv6 addresses follows a structure as shown in Figure 1.
Figure 1: Unique Local IPv6 Address Assignment Structure
The assignment of Global Unicast addresses follows a structure as shown in Figure 2.
Figure 2: Global Unicast IPv6 Address Assignment Structure
The acronyms that are used within Figure 2 include:
- TLA ID– Top-Level Aggregation Identifier
- NLA ID – Next Level Aggregation Identifier
- SLA ID – Site Level Aggregation Identifier