IPv6 Address Allocation and Policy PART1 IPv6 Address Basics

(2004.11.18)

Toshiyuki Hosaka
JPNIC


Computers connected to IP networks are given numbers to identify each of them. This number is called IP address. There are two types of addresses for different IP versions: IPv4 address and IPv6 address. At present, IPv4 address is used dominantly, but increasing number of service providers get IPv6 address allocated to start IPv6 services. More recently, some service providers in Europe and Japan are getting larger address blocks for full-fledged IPv6 services. This series of articles will explain the basics of IPv6 address first, followed by explanation on IPv6 allocation rule management, to tell you the latest situation on IPv6 addresses from the standpoint of an IP address registry.


IPv6 Addressing Scheme

Let me first tell you about IPv6 addressing scheme briefly. IPv6 address is a 128-bit address. By simple arithmetic, about biquadrate of 4.3 billion addresses can be used. This is far more than 32-bit IPv4 address, which theoretically enables 4.3 billion addresses.

IPv4 address is divided to four chunks of 8-bit numbers, each of which is expressed in a decimal number and separated with others by dots. The same expression, if applied to IPv6, would be too long. So IPv6 address is divided to 8 blocks of 16-bit number. Each block is converted to hexadecimal number and separated by colons. One of the additional expression rules is that series of zero can be represented by one zero. As in IPv4, "/" can be used to show prefix length.

IPv4 address expression example 192.168.0.0/24
IPv6 address expression example 2001:0DB8::/32


IPv6 Address Types

IPv6 address, in general, is classified in three types:

(1)	Unicast Address An address applied to one network interface.
(2)	Anycast Address An address applied for multiple network interfaces, but actual communication is conducted with one of them.
(3)	Multicast Address An address applied for multiple network interfaces, and communication is conducted with all hosts with the same address.

There are two types of unicast addresses: aggregatable global unicast address (global IPv6 address) and link-local unicast address (link-local address). IP address registries allocate global IPv6 addresses. The structure of IPv6 address is defined by RFC3587 as in Figure 1.

Figure 1 IPv6 Address Structure 3 bits 45 bits 16 bits 64 bits 001 Global Routing Prefix Subnet ID Interface ID

Specifically, registries are currently allocating addresses with format prefix "001"(2000:/3). Link-local address is used to identify hosts on the same network segment, with a dedicated prefix of "FE80::/10". In the 128-bit address, 16-bit Subnet ID is provided for end user organizations to use for internal network segmentation. The lower 64-bit Interface ID is used to identify network interfaces of hosts. When Interface ID is created through address autoconfiguration, it is configured through EUI-64 rule, that uses MAC address. Therefore, exactly speaking, upper 48-bit of global IPv6 address is subject to hierarchical IPv6 address allocation mechanism. Anycast address is not given a dedicated prefix. Anycast address is defined as a special case of unicast address. Specifically, anycast address has the Interface ID will all zero. Multicast address is given a prefix "FF00::/8", in which the first 8 bits are all 1. 9th bit to 12th bit are used to express the nature of multicast address. If 12th is 0, the multicast address is statically allocated by IANA. And if 11th bit is 1, prefix assigned to the service provider is used. Site Local Address and Unique Local Address IPv4 defines private addresses, such as 10.0.0.0/8 and 192.168.0.0/16, for assignment to hosts which are not directly connected to the Internet. IPv6 had a similar range of address called Site Local Address, with the prefix of "FEC::/10", but it is deprecated now. As a substitute to Site Local Address, standardization of unique local IPv6 unicast address is being discussed at IETF and other related forums, but the work is not finished yet. At an ARIN (American Registry of Internet Numbers) meeting held in October 2004, some expressed concern that such addresses may be used like provider independent address, with uncontrolled route advertisement. Some others agreed to this opinion. Therefore, it is hard to imagine that this discussion will be concluded smoothly. As a summary to the above explanation, let me show you Table 1, an allocation status of IPv6 address format prefix.

Table 1 IPv6 Prefix Allocation Allocation Status Prefix (binary) Prefix (hexadecimal) Ratio to Total IPv6 Address Unallocated*1 [0] 0000 0000 ::/8 1/256 Unallocated 0000 0001 100::/8 1/256 Reserved for NSAP 0000 001 200::/7 1/128 Unallocated 0000 01 400::/6 1/64 Unallocated 0000 1 800::/5 1/32 Unallocated 0001 1000::/4 1/16 Aggregatable Global Unicast Address 001 2000::/3 1/8 Unallocated 010 4000::/3 1/8 Unallocated 011 6000::/3 1/8 Unallocated 100 8000::/3 1/8 Unallocated 101 A000::/3 1/8 Unallocated 110 C000::/3 1/8 Unallocated 1110 E000::/4 1/16 Unallocated 1111 0 F000::/5 1/32 Unallocated 1111 10 F800::/6 1/64 Unallocated*2 [0] 1111 110 FC00::/7 1/128 Unallocated 1111 1110 0 FE00::/9 1/512 Link Local Address 1111 1110 10 FE80::/10 1/1024 Reserved by IANA(was for Site Local Address) 1111 1110 11 FEC0::/10 1/1024 Multicast Address 1111 1111 FF00::/8 1/256 *1:This prefix includes Link Local Address and IPv4-embedded IPv6 Address *2:This prefix might be used for allocation of unique local address being discussed in IETF.

Let me give you the overview of global IPv6 address allocation. IP address has no concept of ownership. It is about the management and use of public resources. Therefore, it is not allowed to sell, buy or give addresses. Address management is conducted in a hierarchical manner, and the top of this hierarchy is IANA (Internet Assigned Numbers Authority). IANA allocates addresses to Regional Internet Registry, or RIR. Currently, there are our RIR: APNIC (Asia Pacific Network Information Centre), ARIN for North America, RIPE NCC (Reseaux IP Europeans Network Coordination Centre) mainly for Europe, and LACNIC (Latin American and Caribbean Internet Addresses Registry) for South America and Caribbean regions. RIR (or NIR) allocates address to organizations called LIR (Local Internet Registry). An LIR is an organization which is delegated by RIR or NIR to allocate address to users. LIR usually is a service provider. An LIR allocates acquired address to end user organizations, or other ISPs. With IPv4, an end user organization may be able to get addresses directly assigned directly in an exceptional case, it the organization meets special criteria. With IPv6, however, RIR (NIR) does not assign global IPv6 address directly to an end user organization, in order to maximize route optimization. Any end user organization gets global IPv6 addresses assigned by the service provider they connect to. If the organization changes the service provider, global routing prefix is inevitably changed. I have used two similar words so far: allocation and assignment. The two words are distinguished in IP address management. Allocation describes distribution of addresses from upstream to downstream organizations for re-distribution. On the other hand, assignment describes distribution of address to end user organizations for their own use. Figure 2: IP Address Management Hierarchy Generally, corporations or individuals get addresses assigned by upstream service providers they are connected. Those who wish to get addresses for re-distribution need to send request for address allocation to RIR/NIR. But they need to meet the requirements in the address allocation policy. The next article explains the currently effective IPv6 address allocation policy, detailing specifically how IPv6 addresses are allocated and assigned.