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The original Internet design called for all organizations to register and be assigned one or
more public IP networks (Class A, B, or C). By registering to use a particular public network
number, the company or organization using that network was assured by the numbering
authorities that no other company or organization in the world would be using the
same addresses. As a result, all hosts in the world would have globally unique IP addresses.
From the perspective of the Internet infrastructure, in particular the goal of keeping Internet
routers’ routing tables from getting too large, assigning an entire network to each organization
helped to some degree. The Internet routers could ignore all subnets as defined
inside an Enterprise, instead having a route for each classful network. For instance, if a
company registered and was assigned Class B network 128.107.0.0/16, the Internet routers
just needed one route for that whole network.
Over time, the Internet grew tremendously. It became clear by the early 1990s that something
had to be done, or the growth of the Internet would grind to a halt when all the
public IP networks were assigned, and no more existed. Additionally, the IP routing tables
in Internet routers were becoming too large for the router technology of that day. So, the
Internet community worked together to come up with both some short-term and longterm
solutions to two problems: the shortage of public addresses and the size of the routing
tables.
The short-term solutions included a much smarter public address assignment policy, in
which public addresses were not assigned as only Class A, B, and C networks, but as
smaller subdivisions (prefixes), reducing waste. Additionally, the growth of the Internet
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routing tables was reduced by smarter assignment of the actual address ranges based on
geography. For example, assigning the class C networks that begin with 198 to only a particular
ISP in a particular part of the world allowed other ISPs to use one route for
198.0.0.0/8–in other words, all addresses that begin with 198–rather than a route for each
of the 65,536 different Class C networks that begin with 198. Finally, NAT/PAT achieved
amazing results by allowing a typical home or small office to consume only one public
IPv4 address, greatly reducing the need for public IPv4 addresses.
IPv6 provides the long-term solution to both problems (address exhaustion and Internet
routing table size). The sheer size of IPv6 addresses takes care of the address exhaustion
issue. The address assignment policies already used with IPv4 have been refined and applied
to IPv6, with good results for keeping the size of IPv6 routing tables smaller in Internet
routers. This section provides a general discussion of both issues, in particular how
global unicast addresses, along with good administrative choices for how to assign IPv6
address prefixes, aid in routing in the global Internet. This section concludes with a discussion
of subnetting in IPv6.
Global Route Aggregation for Efficient Routing
By the time the Internet community started serious work to find a solution to the growth
problems in the Internet, many people already agreed that a more thoughtful public address
assignment policy for the public IPv4 address space could help keep Internet routing
tables much smaller and more manageable. IPv6 public address assignment follows these
same well-earned lessons.
Note: The descriptions of IPv6 global address assignment in this section provides a general
idea about the process. The process may vary from one RIR to another, and one ISP to
another, based on many other factors.
The address assignment strategy for IPv6 is elegant, but simple, and can be roughly summarized
as follows:
■ Public IPv6 addresses are grouped (numerically) by major geographic region.
■ Inside each region, the address space is further subdivided by ISPs inside that region.
■ Inside each ISP in a region, the address space is further subdivided for each customer.
The same organizations handle this address assignment for IPv6 as for IPv4. The Internet
Corporation for Assigned Network Numbers (ICANN, www.icann.org) owns the process,
with the Internet Assigned Numbers Authority (IANA) managing the process. IANA assigns
one or more IPv6 address ranges to each Regional Internet Registries (RIR), of which
there are five at the time of publication, roughly covering North America, Central/South
America, Europe, Asia/Pacific, and Africa. These RIRs then subdivide their assigned address
space into smaller portions, assigning prefixes to different ISPs and other smaller
registries, with the ISPs then assigning even smaller ranges of addresses to their customers.
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The IPv6 global address assignment plan results in more efficient routing, as shown in
Figure 16-1. The figure shows a fictitious company (Company1) which has been assigned
an IPv6 prefix by a fictitious ISP, NA-ISP1 (meaning for North American ISP number 1).
The figure lists the American Registry for Internet Numbers (ARIN), which is the RIR for
North America.
As shown in the figure, the routers installed by ISPs in other major geographies of the
world can have a single route that matches all IPv6 addresses in North America. Although
there might be hundreds of ISPs operating in north America, and hundreds of thousands
of Enterprise customers of those ISPs, and tens of millions of individual customers of
those ISPs, all the public IPv6 addresses can be from one (or a few) very large address
blocks–requiring only one (or a few) routes on the Internet routers in other parts of the
world. Similarly, routers inside other ISPs in North America (for example, NA-ISP2, meaning
North American ISP number 2 in the figure), can have one route that matches all address
ranges assigned to NA-ISP2. And the routers inside NA-ISP1 just need to have one
route that matches the entire address range assigned to Company1, rather than needing to
know about all the subnets inside Company1.
Besides keeping the routers’ routing table much smaller, this process also results in fewer
changes to Internet routing tables. For example, if NA-ISP1 signed a service contract with
another Enterprise customer, NA-ISP1 could assign another prefix inside the range of addresses
already assigned to NA-ISP1 by ARIN. The routers outside NA-ISP1’s
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Company 1
NA-ISP2
NA-ISP1
Europe
South America
1 Route for
All Company 1
Addresses
1 Route for All
NA-ISP1 Addresses
1 Route for All North
American IPv6 Addresses
1 Route for All North
American IPv6 Addresses
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Table 16-2 Hexadecimal/Binary Conversion Chart
Hex Binary Hex Binary
0 0000 8 1000
1 0001 9 1001
2 0010 10 1010
3 0011 11 1011
4 0100 12 1100
5 0101 13 1101
6 0110 14 1110
7 0111 15 1111
network–the majority of the Internet–do not need to know any new routes, because their
existing routes already match the address range assigned to the new customer. The NAISP2
routers (another ISP) already have a route that matches the entire address range assigned
to NA-ISP1, so they do not need any more routes. Likewise, the routers in ISPs in
Europe and South America already have a route that works as well.
Conventions for Representing IPv6 Addresses
IPv6 conventions use 32 hexadecimal numbers, organized into 8 quartets of 4 hex digits
separated by a colon, to represent a 128-bit IPv6 address, for example:
2340:1111:AAAA:0001:1234:5678:9ABC
Each hex digit represents 4 bits, so if you want to examine the address in binary, the conversion
is relatively easy if you memorize the values shown in Table 16-2.
Writing or typing 32 hexadecimal digits, although more convenient writing or typing 128
binary digits, can still be a pain. To make things a little easier, two conventions allow you
to shorten what must be typed for an IPv6 address:
■ Omit the leading 0s in any given quartet.
■ Represent one or more consecutive quartets of all hex 0s with “::” but only for one
such occurrence in a given address.
Note: For IPv6, a quartet is one set of 4 hex digits in an IPv6 address. There are 8 quartets
in each IPv6 address.
For example, consider the following address. The bold digits represent digits in which the
address could be abbreviated.
FE00:0000:0000:0001:0000:0000:0000:0056
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This address has two different locations in which one or more quartets have 4 hex 0s, so
two main options exist for abbreviating this address–using the :: abbreviation in one or the
other location. The following two options show the two briefest valid abbreviations:
FE00::1:0:0:0:56
FE00:0:0:1::56
In particular, note that the “::” abbreviation, meaning “one or more quartets of all 0s,” cannot
be used twice because that would be ambiguous. So, the abbreviation FE00::1::56
would not be valid.
Conventions for Writing IPv6 Prefixes
IPv6 prefixes represent a range or block of consecutive IPv6 addresses. Just like routers
use IPv4 subnets in IPv4 routing tables to represent ranges of consecutive addresses,
routers use IPv6 prefixes to represent ranges of consecutive IPv6 addresses as well. The
concepts mirror those of IPv4 addressing when using a classless view of the IPv4 address.
Figure 16-2 reviews both the classful and classless view of IPv4 addresses, compared to
the IPv6 view of addressing and prefixes.
First, for perspective, compare the classful and classless view of IPv4 addresses. Classful
IPv4 addressing means that the class rules always identify part of the address as the network
part. For example, the written value 128.107.3.0/24 (or 128.107.3.0 255.255.255.0)
means 16 network bits (because the address is in a class B network), 8 host bits (because
the mask has 8 binary 0s), leaving 8 subnet bits. The same value, interpreted with classless
rules, means prefix 128.107.3.0, prefix length 24. Classless addressing and classful addressing
just give slightly different meaning to the same numbers.
Length of Network + Subnet Parts
Network Subnet Host IPv4 Classful Addressing
Host IPv4 Classless Addressing
Host
(Interface ID)
Prefix
Prefix
Prefix Length
Prefix Length
IPv6 Addressing
Figure 16-2 IPv4 Classless and Classful Addressing, IPv6 Addressing
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IPv6 uses a classless view of addressing, with no concept of classful addressing. Like IPv4,
IPv6 prefixes list some prefix value, a slash, and then a numeric prefix length. Like IPv4
prefixes, the last part of the number, beyond the length of the prefix, will be represented
by binary 0’s. And finally, IPv6 prefix numbers can be abbreviated with the same rules as
IPv4 addresses.
Note: IPv6 prefixes are often called IPv6 subnets as well. This book uses these terms
interchangeably.
For example, consider the following IPv6 address that is assigned to a host on a LAN:
2000:1234:5678:9ABC:1234:5678:9ABC:1111/64
This value represents the full 128-bit IP address–there are no opportunities to even abbreviate
this address. However, the /64 means that the prefix (subnet) in which this address resides
is the subnet that includes all addresses that begin with the same first 64 bits as the
address. Conceptually, it is the same logic as an IPv4 address, for example, address
128.107.3.1/24 is in the prefix (subnet) whose first 24 bits are the same values as address
128.107.3.1.
As with IPv4, when writing or typing a prefix, the bits past the end of the prefix length are
all binary 0s. In the IPv6 address previously shown, the prefix in which the address resides
would be
2000:1234:5678:9ABC:0000:0000:0000:0000/64
Which, when abbreviated, would be
2000:1234:5678:9ABC::/64
Next, one last fact about the rules for writing prefixes before seeing some examples and
moving on. If the prefix length is not a multiple of 16, then the boundary between the prefix
and the interface ID (host) part of the address is inside a quartet. In such cases, the prefix
value should list all the values in the last octet in the prefix part of the value. For
example, if the address just shown with a /64 prefix length instead had a /56 prefix length,
the prefix would include all of the first 3 quartets (a total of 48 bits), plus the first 8 bits of
the fourth quartet. The next 8 bits (last 2 hex digits) of the fourth octet should now be binary
0s, as part of the host portion of the address. So, by convention, the rest of the
fourth octet should be written, after being set to binary 0s, as follows:
2000:1234:5678:9A00::/56
The following list summarizes some key points about how to write IPv6 prefixes.
■ The prefix has the same value as the IP addresses in the group for the number of bits
in the prefix length.
■ Any bits after the prefix length number of bits are binary 0s.
■ The prefix can be abbreviated with the same rules as IPv6 addresses.
■ If the prefix length is not on a quartet boundary, write down the value for the entire
quartet.
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Examples can certainly help in this case. Table 16-3 shows several sample prefixes, their
format, and a brief explanation.
Note which options are not allowed. For example, 2::/3 is not allowed instead of 2000::/3,
because it omits the rest of the octet, and a device could not tell if 2::/3 means “hex 0002”
or “hex 2000”.
Now that you understand a few of the conventions about how to represent IPv6 addresses
and prefixes, a specific example can show how IANA’s IPv6 global unicast IP address assignment
strategy can allow the easy and efficient routing shown in Figure 16-1.
Global Unicast Prefix Assignment Example
IPv6 standards reserve the range of addresses inside the 2000::/3 prefix as global unicast
addresses. This address range includes all IPv6 addresses that begin with binary 001, or as
more easily recognized, all IPv6 addresses that begin with a 2 or 3. IANA assigns global
unicast IPv6 addresses as public and globally unique IPv6 addresses, as discussed using
the example previously shown in Figure 16-1, allowing hosts using those addresses to
communicate through the Internet without the need for NAT. In other words, these addresses
fit the purest design for how to implement IPv6 for the global Internet.
Figure 16-3 shows an example set of prefixes that could result in a company (Company1)
being assigned a prefix of 2340:1111:AAAA::/48.
The process starts with IANA, who owns the entire IPv6 address space and assigns the
rights to registry prefix to one of the RIRs (ARIN in this case, in North America). For the
purposes of this chapter, assume that IANA assigns prefix 2340::/12 to ARIN. This assignment
means that ARIN has the rights to assign any IPv6 addresses that begin with the first
12 bits of hex 2340 (binary value 0010 0011 0100). For perspective, that’s a large group of
addresses: 2116 to be exact.
Next, NA-ISP1 asks ARIN for a prefix assignment. After ARIN ensures that NA-ISP1
meets some requirements, ARIN might assign site prefix 2340:1111::/32 to NA-ISP1. This
too is a large group: 296 addresses to be exact. For perspective, this one address block may
1Although an RIR can assign a prefix to an ISP, an RIR may also assign a prefix to other Internet
registries, which might subdivide and assign additional prefixes, until eventually an ISP and
then their customers are assigned some unique prefix.
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