Networking

10 things you should know about IPv6 addressing

Although IPv6 adoption seems to be moving at a snail's pace, there's no outrunning it. Brien Posey demystifies some of the addressing issues many admins are still trying to figure out.
[Editor's note: This article has been revised to correct a couple of errors noted by TechRepublic members. Thanks to everyone who contributed their input.]

Over the last several years, IPv6 has been inching toward becoming a mainstream technology. Yet many IT pros still don't know where to begin when it comes to IPv6 adoption because IPv6 is so different from IPv4. In this article, I'll share 10 pointers that will help you understand how IPv6 addressing works.

1: IPv6 addresses are 128-bit hexadecimal numbers

The IPv4 addresses we are all used to seeing are made up of four numerical octets that combine to form a 32-bit address. IPv6 addresses look nothing like IPv4 addresses. IPv6 addresses are 128 bits in length and are made up of hexadecimal characters.

In IPv4, each octet consists of a decimal number ranging from 0 to 255. These numbers are typically separated by periods. In IPv6, addresses are expressed as a series of eight 4-character hexadecimal numbers, which represent 16 bits each (for a total of 128 bits). As we'll see in a minute, IPv6 addresses can sometimes be abbreviated in a way that allows them to be expressed with fewer characters.

2: Link local unicast addresses are easy to identify

IPv6 reserves certain headers for different types of addresses. Probably the best known example of this is that link local unicast addresses always begin with FE80. Similarly, multicast addresses always begin with FF0x, where the x is a placeholder representing a number from 1 to 8.

3: Leading zeros are suppressed

Because of their long bit lengths, IPv6 addresses tend to contain a lot of zeros. When a section of an address starts with one or more zeros, those zeros are nothing more than placeholders. So any leading zeros can be suppressed. To get a better idea of what I mean, look at this address:

FE80:CD00:0000:0CDE:1257:0000:211E:729C

If this were a real address, any leading zero within a section could be suppressed. The result would look like this:

FE80:CD00:0:CDE:1257:0:211E:729C

As you can see, suppressing leading zeros goes a long way toward shortening the address.

4: Inline zeros can sometimes be suppressed

Real IPv6 addresses tend to contain long sections of nothing but zeros, which can also be suppressed. For example, consider the address shown below:

FE80:CD00:0000:0000:0000:0000:211E:729C

In this address, there are four sequential sections separated by zeros. Rather than simply suppressing the leading zeros, you can get rid of all of the sequential zeros and replace them with two colons. The two colons tell the operating system that everything in between them is a zero. The address shown above then becomes:

FE80:CD00::211E:729C

You must remember two things about inline zero suppression. First, you can suppress a section only if it contains nothing but zeros. For example, you will notice that the second part of the address shown above still contains some trailing zeros. Those zeros were retained because there are non-zero characters in the section. Second, you can use the double colon notation only once in any given address.

5: Loopback addresses don't even look like addresses

In IPv4, a designated address known as a loopback address points to the local machine. The loopback address for any IPv4-enabled device is 127.0.0.1.

Like IPv4, there is also a designated loopback address for IPv6:

0000:0000:0000:0000:0000:0000:0000:0001

Once all of the zeros have been suppressed, however, the IPv6 loopback address doesn't even look like a valid address. The loopback address is usually expressed as ::1.

6: You don't need a traditional subnet mask

In IPv4, every IP address comes with a corresponding subnet mask. IPv6 also uses subnets, but the subnet ID is built into the address.

In an IPv6 address, the first 48 bits are the network prefix. The next 16 bits are the subnet ID and are used for defining subnets. The last 64 bits are the interface identifier (which is also known as the Interface ID or the Device ID).

If necessary, the bits that are normally reserved for the Device ID can be used for additional subnet masking. However, this is normally not necessary, as using a 16-bit subnet and a 64-bit device ID provides for 65,535 subnets with quintillions of possible device IDs per subnet. Still, some organizations are already going beyond 16-bit subnet IDs.

7: DNS is still a valid technology

In IPv4, Host (A) records are used to map an IP address to a host name. DNS is still used in IPv6, but Host (A) records are not used by IPv6 addresses. Instead, IPv6 uses AAAA resource records, which are sometimes referred to as Quad A records. The domain ip6.arpa is used for reverse hostname resolution.

8: IPv6 can tunnel its way across IPv4 networks

One of the things that has caused IPv6 adoption to take so long is that IPv6 is not generally compatible with IPv4 networks. As a result, a number of transition technologies use tunneling to facilitate cross network compatibility. Two such technologies are Teredo and 6to4. Although these technologies work in different ways, the basic idea is that both encapsulate IPv6 packets inside IPv4 packets. That way, IPv6 traffic can flow across an IPv4 network. Keep in mind, however, that tunnel endpoints are required on both ends to encapsulate and extract the IPv6 packets.

9: You might already be using IPv6

Beginning with Windows Vista, Microsoft began installing and enabling IPv6 by default. Because the Windows implementation of IPv6 is self-configuring, your computers could be broadcasting IPv6 traffic without your even knowing it. Of course, this doesn't necessarily mean that you can abandon IPv4. Not all switches and routers support IPv6, just as some applications contain hard-coded references to IPv4 addresses.

10: Windows doesn't fully support IPv6

It's kind of ironic, but as hard as Microsoft has been pushing IPv6 adoption, Windows does not fully support IPv6 in all the ways you might expect. For example, in Windows, it is possible to include an IP address within a Universal Naming Convention (\\127.0.0.1\C$, for example). However, you can't do this with IPv6 addresses because when Windows sees a colon, it assumes you're referencing a drive letter.

To work around this issue, Microsoft has established a special domain for IPv6 address translation. If you want to include an IPv6 address within a Universal Naming Convention, you must replace the colons with dashes and append .ipv6.literal.net to the end of the address — for example, FE80-AB00�D-617B.ipv6.literal.net.

Note: This article is also available as a PDF download.


About

Brien Posey is a seven-time Microsoft MVP. He has written thousands of articles and written or contributed to dozens of books on a variety of IT subjects.

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