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IPv6 addresses are different from IPv4 addresses in far more ways than just their length. The “shorthand” for writing them is different, they have significantly different formats, and their functional organization is different. This section introduces you to those differences.
You certainly already know that 32-bit IPv4 addresses are represented by breaking them into four 8-bit segments and writing each of those segments in decimal between 0 and 255, separating them with periods; hence the term dotted decimal.
128-bit IPv6 addresses are represented by breaking them up into eight 16-bit segments. Each segment is written in hexadecimal between 0×0000 and 0xFFFF, separated by colons. An example of a written IPv6 address is
Remembering more than a few such addresses is practically impossible, and writing them is not much fun either. Fortunately, there are two rules for reducing the size of written IPv6 addresses. The first rule is
The leading zeroes in any 16-bit segment do not have to be written; if any 16-bit segment has fewer than four hexadecimal digits, it is assumed that the missing digits are leading zeroes.
In the example address, the third, fourth, fifth, sixth, and eighth segments have leading zeroes. Using the first address compression rule, the address can be written as
Notice that only leading zeroes can be omitted; trailing zeroes cannot, because doing so would make the segment ambiguous. You would not be able to tell whether the missing zeroes belonged before or after the written digits.
Notice also that the fifth segment in the example address is all zeroes, and is written with a single zero. Many IPv6 addresses have long strings of zeroes in them. Take, for example, the following address:
This address can be reduced as follows:
However, using the second rule can reduce this address even further:
Any single, contiguous string of one or more 16-bit segments consisting of all zeroes can be represented with a double colon.
Using this rule, the example address can be represented as the following:
The increased convenience in writing such an address is obvious. But notice that the rule says only a single contiguous string of all-zero segments can be represented with a double colon. Using the double colon more than once in an IPv6 address can create ambiguity. Take, for example, the following address:
Either of the following reductions of the address is correct because they use a double colon only once:
But the following reduction is illegal because it uses the double colon twice:
It is illegal because the length of the two all-zero strings is ambiguous; it could represent any of the following IPv6 addresses:
Unlike IPv4, in which the prefixthe network portion of the addresscan be identified by a dotted decimal or hexadecimal address mask or a bitcount, IPv6 prefixes are always identified by bitcount. That is, the address is followed by a forward slash and a decimal number indicating how many of the first bits of the address are the prefix bits. For example, the prefix of the following address is the first 64 bits:
When you are writing just an IPv6 prefix, you set all the host bits to 0 the same way you do with IPv4 addresses. For example
An IPv6 address consisting of all zeroes can be written simply with a double colon. There are two cases where an all-zeroes address is used. The first is a default address, discussed in Chapter 12, “Default Routes and On-Demand Routing,” in which the address is all zeroes and the prefix length is zero:
The second all-zeroes IPv6 address is an unspecified address, which is used in some Neighbor Discovery Protocol procedures described later in this chapter. An unspecified address is a filler, indicating the absence of a real IPv6 address. When writing an unspecified address, it is differentiated from a default address by its prefix length:
The three types of IPv6 address follow:
Unlike IPv4, there is no IPv6 broadcast address. There is, however, an “all nodes” multicast address, which serves essentially the same purpose as a broadcast address.
A unicast address is an address that identifies a single device. A global unicast address is a unicast address that is globally unique. The general format of the IPv6 unicast address is shown in Figure 2-1. This format, specified in RFC 3587, obsoletes and simplifies an earlier format that divided the IPv6 unicast address into Top Level Aggregator (TLA), Next-Level Aggregator (NLA), and other fields. However, you should be aware that this obsolescence is relatively recent and you are likely to encounter some books and documents that show the old IPv6 address format.
The host portion of the address is called the Interface ID. The reason for this name is that a host can have more than one IPv6 interface, and so the address more correctly identifies an interface on a host than a host itself. But that subtlety only goes so far: A single interface can have multiple IPv6 addresses, and can have an IPv4 address in addition, in which case the Interface ID is only one of that interface’s several identifiers.
Perhaps the most striking difference between IPv4 addresses and IPv6 addresses, aside from their lengths, is the location of the Subnet Identifier as a part of the network portion of the address rather than the host portion. A legacy of the IPv4 address class architecture is that the subnet portion of an IPv4 address is taken from the host portion of the address. As a result, the host portion of the IPv4 address varies not only with its class, but also with the number of bits you use for subnet identification.
The immediate benefit of making the IPv6 Subnet ID field a part of the network portion of the address is that the Interface ID can be a consistent size for all IPv6 addresses, simplifying the parsing of the address. And making the Subnet ID a part of the network portion creates a clear separation of functions: The network portion provides the location of a device down to the specific data link and the host portion provides the identity of the device on the data link.
The Interface ID of the global IPv6 address is, with very few exceptions, 64 bits long. Also with very few exceptions, the Subnet ID field is 16 bits (Figure 2-2). A 16-bit Subnet ID field provides for 65,536 separate subnets; it seems that using a fixed Subnet ID size such as this, when in most cases the capacity will not be nearly fully used, is wasteful. But given the overall size of the IPv6 address space, and given the benefits of easy address assignment, design, management, and parsing that comes from using a fixed size, the waste is justified.
The IANA and the Regional Internet Registries (RIRs) assign IPv6 prefixesnormally /32 or /35 in lengthto the Local Internet Registries (LIRs). The LIRs, which are usually large Internet Service Providers, then allocate longer prefixes to their customers. In the majority of cases, the prefixes assigned by the LIRs are /48. There are, however, as mentioned in the previous paragraph, a few exceptions in which the LIR might assign a prefix of a different length:
 As of this writing there are five RIRs: Réseaux IP Européens (RIPE) serves Europe, the Middle East, and Central Asia; Latin American and Caribbean Internet Address Registry (LACNIC) serves Central and South America and the Caribbean; American Registry for Internet Numbers (ARIN) serves North America and parts of the Caribbean; AfriNIC serves Africa; and Asia Pacific Network Information Centre serves Asia and the Pacific Ocean nations.
The first few bits of the address specify the address type. For example, the first three bits of all global unicast addresses currently are 001. As a result, recognizing the hexadecimal representations of global unicast addresses is fairly easy: They all start with either 2 or 3, depending on the value of the fourth bit in the global routing prefix. So, for instance, currently allocated prefixes used by the 6Bone (the public IPv6 research network) begin with 3ffe, and IPv6 addresses currently allocated by the RIRs begin with 2001.
Binary 001 is expected to suffice for global unicast addresses for some time to come; a few other bit combinations are assigned to other defined address types, and the majority of leading bit combinations are reserved. Table 2-1 lists the currently allocated leading bit combinations, and the following subsections describe the other major IPv6 address types.
High-Order Bits (binary)
High-Order Bits (Hex)
Site-Local Unicast (Deprecated)
Global Unicast (Currently allocated)
2xxx::/4 or 3xxx::/4
Reserved (Future global unicast allocations)
When we talk of global unicast addresses, we mean an address with global scope. That is, an address that is globally unique and can therefore be routed globally with no modification.
IPv6 also has a link-local unicast address, which is an address whose scope is confined to a single link. Its uniqueness is assured only on one link, and an identical address might exist on another link, so the address is not routable off its link. As you can see in Table 2-1, the first 10 bits of the link-local unicast address are always 1111111010 (FF80::/10).
As subsequent sections in this chapter demonstrate, link-local addresses have great utility for functions such as the Neighbor Discovery Protocol that communicates only on a single link. It also allows devices that are on links that do not have assigned global prefixes, or devices that do not yet know the global prefix assigned to the link, to create IPv6 addresses that allow them to communicate with other devices on the link. The section “Address Autoconfiguration” shows how link-local prefixes are used in this situation.
IPv6 originally defined a site-local unicast address in addition to the link-local address. A site-local address is unique only within a given site; devices in other sites can use the same address. Therefore a site-local address is routable only within the site to which it is assigned. Site-local IPv6 addresses are, then, functionally similar to private IPv4 addresses as defined in RFC 1918.
Advocates of site-local addresses cite several applications. One prominent application is for network operators that wish to use NAT, even with IPv6 addresses, to maintain independence of their address architecture from that of their service providers. Site-local addresses are also key to several proposed IPv6 multihoming mechanisms.
However, the IETF IPv6 Working Group determined that site-local unicast addresses introduced a number of difficulties. Not the least of the difficulties is the fact that the definition of a “site” is vague and can mean different things to different network administrators. Another problem is concern over, like RFC 1918 IPv4 addresses, the administrative difficulties introduced when such addresses are mistakenly “leaked” outside of their intended site boundaries. Other potential problems cited include increased complexity for applications and routers that must recognize and cope with site-local addresses. As a result of these concerns, and after some heated debate, the IPv6 Working Group deprecated site-local addresses in RFC 3879. An assurance has been given to those who see advantages in site-local addresses to introduce another scheme with similar “bigger scope than link but smaller scope than global” benefits, but as of this writing such a replacement scheme has yet to be seen.
The first 10 bits of site-local unicast addresses, as shown in Table 2-1, is 1111111011 (FFC0::/10).
An anycast address represents a service rather than a device, and the same address can reside on one or more devices providing the same service. In Figure 2-3, some service is offered by three servers, all advertising the service at the IPv6 address 3ffe:205:1100::15. The router, receiving advertisements for the address, does not know that it is being advertised by three different devices; instead, the router assumes that it has three routes to the same destination and chooses the lowest-cost route. In Figure 2-3 this is the route to server C with a cost of 20.
The advantage of anycast addresses is that a router always routes to the “closest” or “lowest-cost” server. So servers providing some commonly used service can be spread across a large network and traffic can be localized or scoped to the nearest server, making traffic patterns in the network more efficient. And if one server becomes unavailable, the router routes to the next nearest server. In Figure 2-3, for example, if server C becomes unavailable due to a network or server failure, the router chooses the path to server A as the next-lowest-cost route. From the router’s viewpoint, it is just choosing the next-best route to the same destination.
 The methods by which a router chooses among multiple routes to the same destination is covered in Chapter 4, “Dynamic Routing Protocols.
Anycast addresses are defined by their service function only, not by format, and theoretically might be any IPv6 unicast address of any scope. However, there is a format for reserved anycast addresses, defined in RFC 2526. Anycast addresses have been used for some time in IPv4 networks, but are formalized in their definition in IPv6.
A multicast address identifies not one device but a set of devicesa multicast group. A packet being sent to a multicast group is originated by a single device; therefore a multicast packet normally has a unicast address as its source address and a multicast address as its destination address. A multicast address never appears in a packet as a source address.
The members of a multicast group might include only a single device, or even all devices in a network. In fact, IPv6 does not have a reserved broadcast address like IPv4, but it does have a reserved all-nodes multicast group, which is essentially the same thing: a multicast group to which all receiving devices belong.
Multicasting is essential to the basic operation of IPv6, particularly some of its plug-and-play features such as router discovery and address autoconfiguration. These functions are a part of the Neighbor Discovery Protocol, discussed later in this chapter.
The format of the IPv6 multicast address is shown in Figure 2-4. The first eight bits of the address are always all ones, and the next four bits are designated as flags. Currently the first three of these bits are unused and always set to 0. The fourth bit indicates whether the address is a permanent, well-known address (0) or an administratively assigned transient address (1). The next four bits indicate the scope of the address as shown in Table 2-2. Table 2-3 shows several reserved, well-known IPv6 multicast addresses, all of which are link-local scope. Because a multicast group is always a set of individual nodes, there is no needor sensefor having a subnet field in the multicast address. So the last 112 bits are used as the Group-ID, identifying individual multicast groups. Current usage sets the first 80 bits to 0 and just uses the last 32 bits.
Scope Field Value
OSPFv3 Designated Routers
DHCP Servers/Relay Agents
All PIM Routers
There are several transition technologiesmeans of helping to transition a network from IPv4 to IPv6 or otherwise help IPv4 and IPv6 to coexistthat require an IPv4 address to be communicated within an IPv6 address. The individual technology specifies how the IPv4 address is to be embedded in the IPv6 address, and the implementation of the technology knows where among the 128 bits of the IPv6 address to find the 32 bits of the IPv4 address. But you will also find that many of these technologies have unique formats for their address representations that allow you to identify the embedded IPv4 address. Examples of IPv6 addresses with an embedded IPv4 address of 10.23.1.5 are
FE80::5EfE:10.23.1.5 (An ISATAP address)
::FFFF:10.23.1.5 and ::FFFF:0:10.23.1.5 (SIIT addresses)
FEC0:0:0:1::10.23.1.5 (TRT address)
In each of these examples, the IPv4 address is the last 32 bits of the IPv6 address and is represented in dotted decimal.
Other transition technologies using embedded IPv4 addresses do not use dotted decimal but encode the IPv4 address into hexadecimal. 6to4, for example, does this. 10.23.1.5 in hexadecimal is 0A17:0105. A 6to4 prefix with 10.23.1.5 embedded is then
Transition technologies are not covered in this volume, and so you are not likely to see one of these address representations again in this book. They are shown here only because you are likely to encounter addresses like these if you work with IPv6.
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