Global Unicast Addresses make up the bulk of the IPv6 address space, with a little of that address space allocated for Unique Local Unicast addresses which begin with Hex FD.
Global Unicast addresses comprise a 48-bit Global Routing Prefix, 16-bit Subnet ID and an Interface ID comprising the remaining 64 bits. Current Global Unicast addresses assigned by the IANA use the range of addresses beginning binary value 001, which in hexadecimal begin with 2000::/3, which is 1/8 of the total IPv6 address space.
Global Unicast Addresses are broken down as follows:
48 bit Global Routing Prefix
16 bit Subnet ID
64 bit Interface ID
Local organisations can use the 16 bit Subnet ID to create their own local subnets, which would allow an organisation the ability to create 65,535 unique subnets.
Subnetting of IPv6 addresses is similar in makeup to that of IPv4, in the fact that both have a Subnet Prefix at the front end of the address and a Host or Interface ID section at the rear of the address. Although Prefix Lengths can differ, a 64-bit prefix or /64 has been adopted by most organisations. Analyzing IPv6 Global Routing Prefixes is an uncomplicated fairly easy affair when compared to that of IPv4. No separate dotted decimal mask to deal with, as all the information is contained within the IP Address with its Prefix Length.
IPv4 requires Subnets for all LANs and VLANs within LANs, for all Point to Point WAN links and in fact any WAN areas of the internetwork.. With IPv4 it is achieved by manipulating the IPv4 Subnet mask. IPv6 also requires Subnets for all the network components listed above and it is achieved by using 16 bits following the 48 bit Global Routing Prefix issued by the Regional Internet Registries. The remaining 64 bits are used to represent a physical Interface ID.
The Global Routing Prefix would be allocated by an ISP or Regional Internet Registry such as RIPE in Europe or ARIN in North America. A company would receive a Global Routing Prefix and a Prefix Length, the latter determining the potential number of Host Interfaces that could be assigned a unique IPv6 Address. The interface ID is 64 bits in length, but does not have to be this length. We could have a larger Subnet section and a smaller Interface ID, but most organisations have settled for a 64 bit Interface ID and it has almost become the convention. With IPv6 we assign a Prefix, let us say /48, so the first 48 bits of the address are the Global Routing Prefix, 16 bits for Subnet IDs and the remaining bits are referred to as the Interface ID which will provide a unique reference to a Host Interface.
To give an example, a Regional Internet Registry has issued a Global Routing Prefix of 2001:0B6E:2222 to an organisation with a Prefix Length of /48. It is really up to the organisation how it allocates the remaining bits. In this instance, a network administrator has allocated 16 Subnet bits with Hexadecimal 0001, which provides for up to 216 Subnets each with 264 Host Interface IDs. Using 16 and 64 for Subnet ID and Interface ID just keeps the maths simple, but the administrator could have chosen 20 bits or 24 bits for the Subnet ID. With 216 or 65535 Subnets.
Routers list the Subnet ID, otherwise known as the Prefix ID in their Routing Tables alongside the Prefix Length, so in this example we would have 2001:0B6E:2222:1::/64, where 2001:0B6E:2222 is the Global Prefix and 0011, abbreviated 1 is the subnet ID.
If you use a single Prefix Length for all your Subnets as most organisations do, then starting with the Global Routing Prefix you can write down all Subnet IDs in the Subnet Field using Hexadecimal notation.
The Subnet IDs will all use the same Global Routing Prefix
Each Subnet will have a unique value in the Subnet Field
The Interface ID will be all 0s for each Subnet.
Let us calculate the first few Subnet IDs from the following Global Routing Prefix assuming we are using the fourth quartet for Subnet IDs:
2001:07A2:1111::/48 using the fourth quartet for Subnet IDs:
Remember when you start to increment the Subnet ID numbers you are not counting in Binary but in Hexadecimal, so the IDs go from 1 – 9 then A – F and then to the next column. It is that simple!
This article was written by David Christie, MD at NSTUK Ltd, who specialise in the delivery of Instructor-Led training courses including Internet Protocol and Voice over IP training courses. Visit www.nstuk.com/training.html