DNS Appendix
Resource Record Classes
HS = hesiod
The [hesiod] class is an information service developed by MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, printers and so on. The keyword hs is a synonym for hesiod.
CH = chaos
The chaos class is used to specify zone data for the MIT-developed CHAOSnet, a LAN protocol created in the mid-1970s.
General DNS Reference Information
A.3.1. IPv6 addresses (A6)
IPv6 addresses are 128-bit identifiers for interfaces and sets of interfaces which were introduced in the DNS to facilitate scalable Internet routing. There are three types of addresses: Unicast, an identifier for a single interface; Anycast, an identifier for a set of interfaces; and Multicast, an identifier for a set of interfaces. Here we describe the global Unicast address scheme. For more information, see RFC 2374.
The aggregatable global Unicast address format is as follows:
3 13 8 24 16 64 bits FP TLA ID RES NLA ID SLA ID Interface ID <------ Public Topology ------>
<-Site Topology-> <------ Interface Identifier ------>
Where
FP = Format Prefix (001) TLA ID = Top-Level Aggregation Identifier RES = Reserved for future use NLA ID = Next-Level Aggregation Identifier SLA ID = Site-Level Aggregation Identifier INTERFACE ID = Interface Identifier
The Public Topology is provided by the upstream provider or ISP, and (roughly) corresponds to the IPv4 network section of the address range. The Site Topology is where you can subnet this space, much the same as subnetting an IPv4 /16 network into /24 subnets. The Interface Identifier is the address of an individual interface on a given network. (With IPv6, addresses belong to interfaces rather than machines.)
The subnetting capability of IPv6 is much more flexible than that of IPv4: subnetting can now be carried out on bit boundaries, in much the same way as Classless InterDomain Routing (CIDR).
The internal structure of the Public Topology for an A6 global unicast address consists of:
3 13 8 24 FP TLA ID RES NLA ID A 3 bit FP (Format Prefix) of 001 indicates this is a global Unicast address. FP lengths for other types of addresses may vary.
13 TLA (Top Level Aggregator) bits give the prefix of your top-level IP backbone carrier.
8 Reserved bits
24 bits for Next Level Aggregators. This allows organizations with a TLA to hand out portions of their IP space to client organizations, so that the client can then split up the network further by filling in more NLA bits, and hand out IPv6 prefixes to their clients, and so forth.
There is no particular structure for the Site topology section. Organizations can allocate these bits in any way they desire.
The Interface Identifier must be unique on that network. On ethernet networks, one way to ensure this is to set the address to the first three bytes of the hardware address, "FFFE", then the last three bytes of the hardware address. The lowest significant bit of the first byte should then be complemented. Addresses are written as 32-bit blocks separated with a colon, and leading zeros of a block may be omitted, for example:
3ffe:8050:201:9:a00:20ff:fe81:2b32
IPv6 address specifications are likely to contain long strings of zeros, so the architects have included a shorthand for specifying them. The double colon (`::') indicates the longest possible string of zeros that can fit, and can be used only once in an address.