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Windows Server 2012 : IPv6 Introduction (part 3) - Comprehending IPv6 Addressing

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3/15/2015 5:30:08 AM

Learning IPv6 notation can be a bit daunting at first, based not only on the use of hexadecimal numbering, but also on the length of a full address. Using notational abbreviations, as described in RFCs 4291, 5952, and 6052, can assist greatly, but hands-on practice implementing IPv6 addressing and gaining familiarity with binary, decimal, and hexadecimal conversion is key to adopting this new protocol. This section covers addition concepts that network administrators need to know, beyond just the IPv6 address itself.

IPv6 Address Prefix

IPv4 address notation is usually written with a 32-bit network address followed by a forward slash (/) and a trailing number used to delineate the network number and the associated subnet mask. This notation tells the network administrator the range of IP addresses available on that particular network segment. For example, the 192.168.1.0/24 network includes a network number of 192.168.1.0 with a 24-bit subnet mask of 255.255.255.0. To understand what that means, remember that everything goes back to binary, and each binary digit represents 1 bit of the address. Binary addition, which you should become familiar with if you are not already, defines that any number added with 1 will equal 1 and any number added with 0 will equal 0. To understand IP addressing, network administrators need to fully understand these binary addition rules. So, moving forward, the term mask as it relates to IP networking means performing binary addition of a network address with a predetermined bit length of all 1s. For example, a /24 in an 32-bit IPv4 address includes 24 leading 1s and 8 trailing 0s. Table 2 shows how binary addition is used on an existing address of 192.168.1.15, with a subnet mask of 255.255.255.0, to derive the network number.

Table 2. Binary Addition

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Table 2 displays the network number that was derived from addition of the IPv4 address and the associated subnet mask. Although IPv6 has changed the terminology some, this example still applies.

In IPv4, the address is divided into the network number and the host number. In IPv6, the network number portion of the address is referred to as the IPv6 prefix. The IPv6 prefix is written in the same form as for an IPv4 network, but with an IPv6 address followed by a forward slash and a trailing number that is simply referred to as the mask length and written as IPv6address/prefix-length. For example, 2001:dba:1234:5678::/64 translates into a network with a mask of 64 bits, and the IP addresses in this network will all start with the prefix of 2001:dba:1234:5678, followed by four other groups that define the host portion of the address. This example made use of an IPv6 notation reduction that truncated the leading 0 in the second group of dba. 

IPv6 DNS Records

Because IPv6 addresses use a completely different number scheme, a new DNS record format was created to support the lookup of IPv6 hosts. The IPv6 forward, or name to IPv6 address type, is an AAAA address type. This address type is available on Microsoft Windows DNS servers and on most other current DNS server systems. For an organization that will leverage IPv6, network and DNS administrators need to ensure that their DNS server system supports this record type. The same goes for the reverse DNS record, where the IPv6 address lookup needs to resolve to a hostname. With Microsoft DNS, the forward lookup zone can support both the standard IPv4 record type of A and the IPv6 record type of AAAA. The reverse DNS zone must be created and populated manually, but even though there will need to be separate reverse DNS zones for IPv4 and IPv6 networks, the reverse DNS records for both are still the same PTR record type. Microsoft clients and servers will, by default, dynamically register their forward and reverse IPv4 addresses in DNS. With IPv6, only the forward DNS records are registered, and if network administrators require IPv6 reverse DNS records, these must be created manually when needed.

IPv6 Address Notation Best Practices

As stated earlier, IPv6 addresses are long, and RFCs have been written to provide standards and best practices for administrators to follow to simplify notation, when possible. Some of the most common IPv6 character-reduction practices are as follows and are all part of what is referred to as 0 compression:

Truncate leading 0s—Whenever possible, remove one or more leading 0s in an IPv6 address group (for example, by changing the IPv6 address of 2001:0dba:0000:0000:0000:0110:0000:0000 to 2001:dba:0000:0000:0000:110:0000:0000)

Compress 0 groups—When an entire group of an IPv6 address is made up of all 0s (for example, 2001:dba:0000:0000:0000:110:0000:0000), the group can be reduced to a single 0 (2001:dba:0:0:0:110:0:0).

Replace consecutive zero groups with double colon—When consecutive 0 groups are encountered, as shown in the previous example, two or more groups can be replaced with a double colon.

• Only one set of double colons can be used in an IPv6 address.

• When there is more than one set of zero groups in an IPv6 address, the double colon should be used to replace the largest set. For this example, that is the last three groups of 0s; the IPv6 address is then written as 2001:dba::110:0:0.

• When multiple consecutive groups of 0s are replaced with a double colon, administrators can subtract the remaining number of groups from 8 to determine how many groups of 0s were removed. In this case, 8 – 5 = 3, so the double colon has replaced three groups of consecutive 0s.

Do not remove trailing or 0s between other numbers—When a group contains a 0 that is surrounded by other numbers or is the last number in a group, it needs to remain as part of the final address. For example, in the sixth group (originally 0110), removing the trailing 0 would change the number and therefor change the address. This might seem obvious, but with all the available options to reduce the address, it is important to note this rule.

So with all the options previously noted for 0 compression, the original address of 2001:0dba:0000:0000:0000:0110:0000:0000 is now reduced to 2001:dba::110:0:0, and this reduces the original 39 character address to 17 characters (including colons).

One more important point about address notation is that although it is not a standard, a good practice for IPv6 notation is to use all lowercase letters.

IPv6 Addresses and Port Specification

In many documentation cases, it is necessary to also designate the port of a service running on IPv6. IPv4 uses a colon to designate the port (for example, 192.168.1.15:80), but with IPv6 this would cause a lot of confusion. To designate an IPv6 and a port address, you can enclose the address in brackets followed by the colon and port number (for example, [2001:dba::110:0:0]:80); 2001:dba::110:0:0 port 80 is also acceptable.

Unique Local Address

As with IPv4, many organization use the designated IPv4 private IP address ranges of 10.0.0.0/8, 172.16.0.0/16, and 192.168.0.0/24. With IPv6, because there are enough addresses, organizations may not need to use private addressing with NAT at the router or perimeter firewall. Organizations may start to use globally routable IPv6 addresses, or addresses that can be used on the internal network and on the Internet. However, many network administrators may opt to start IPv6 implementations using an internal-use IPv6 address known as the unique local address (ULA). This the IPv6 counterpart to the IPv4 private IP ranges and is detailed in RFC 4193. The ULA prefix is fc00::/7. The addresses within this range are intended to be used on internal site networks, and can even be used to route between internal networks, but are not intended to be routed globally or on the Internet.

The fc00::/7 prefix is divided further into two /8 address prefixes, including fd00:/8, which should be used for actual network deployments and has the eighth bit set to 1, indicating a local network, followed by a randomly generated 40-bit string, known as the global ID and a 16-bit subnet ID allowing network administrators a standard for addressing devices on /64 subnets. The other network is fc00:/8 and has the eighth bit set to 0, but that network has not yet been clearly defined or accepted as a standard. One network range from the fd00::/8 prefix is the fe80::/64 prefix, which is used for link-local addresses. The link-local address is the self-generated address assigned on every IPv6-enabled adapter. This link-local address can be used between IPv6 devices that were not statically configured with an address and did not receive an address from an IPv6 DHCP server (that is, the stateless configuration IPv6 address).

 
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