10 things you should know about IPv6 addressing

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.

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

1: IPv6 addresses are 128-bit Hexadecimal numbers

The IPv4 addresses we’re 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 long and are made up of hexadecimal numbers.

In IPv4, each octet is separated by a period. In IPv6, the hexadecimal characters are separated by colons. A group of hexadecimal characters can range from two to four characters in length.

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 characters are the network prefix. The next 16 characters (which are often all zeros) are the subnet ID. And the last 64 characters are the interface identifier. Even though there is no subnet mask, you have the option of specifying a subnet prefix length.

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-AB00–200D-617B.ipv6.literal.net.