IPv6 Part 3: Address auto-configuration

In the previous post I explained that network prefixes in IPv6 are nearly always a fixed 64 bits in length, with a few exceptions. Strictly speaking, it’s actually the interface ID that’s fixed at 64 bits, which means that because IPv6 addresses are 128 bits long the network prefix has to be 64 bits too. The reason for this is to meet the needs of address auto-configuration, which is a new feature of IPv6 that supports:

  • link-local address auto-configuration (so-called plug-and-play)
  • stateless address auto-configuration (SLAAC, for global addresses)

Link-local addressing is pretty fundamental to IPv6. It enables a host to communicate with other nodes on the same network without the need for manual configuration or a DHCP server. The process is as follows:

  1. In the conventional method the host generates a 64-bit identifier for the interface by taking its 48-bit MAC address and inserting the string fffe into the middle. Then it “flips” bit 7 of the address from 0 to 1, so that the MAC address:


    generates the interface ID:


    This is what’s known as a Modified EUI-64 identifier (there are other methods but I will cover those in a later post).

  2. The host prefixes this identifier with the link-local network prefix fe80::/64, so that our example MAC address generates the link-local address:


  3. The host then tests this address for uniqueness on the local subnet by sending a Neighbor Solicitation message (ICMPv6 type 135) to the generated address. If it gets a Neighbor Advertisement message (ICMPv6 type 136) in reply then the process halts.
  4. If there is no response then the link-local address is assigned to the interface.

IPv6 link-local addressing

You can find more detail on this at Packetlife.

Once the link-local address is configured, the host sends a Router Solicitation message (ICMPv6 type 133) to the All Local Routers multicast address ff02::2. If there’s a local IPv6 router it will respond with a Router Advertisement message (ICMPv6 type 134). If that tells the host that it should auto-configure a global address then:

  1. The host will take the 64-bit global prefix advertised by the router and combine it with the 64-bit interface identifier previously generated to create a global address, so if the prefix is:


    then our example interface ID generates the global address:


  2. It will test for uniqueness on the local subnet as for the link-local address
  3. If there is no response then the global address is assigned to the interface.


The whole set of messages and processes, together with other ancilliary messages and processes, is known as Neighbor Discovery. In principle other methods of global addressing (manual, DHCP) could use a variable-length prefix. However the address architecture RFC (RFC 4291) stipulates a 64-bit interface ID regardless of the configuration method. RFC 5375 warns against straying from the /64 prefix:

Using a subnet prefix length other than a /64 will break many features of IPv6, including Neighbor Discovery (ND), Secure Neighbor Discovery (SEND) [RFC3971], privacy extensions [RFC4941], parts of Mobile IPv6 [RFC4866], Protocol Independent Multicast – Sparse Mode (PIM-SM) with Embedded-RP [RFC3956], and Site Multihoming by IPv6 Intermediation (SHIM6) [SHIM6], among others.

In fact it’s specifically the SLAAC component of Neighbor Discovery that would be broken by a global network prefix other than /64, because of the way that the interface ID is generated.

In the next post, I’ll delve more into the background of SLAAC.

Yes it’s a lot of bits but…

Let’s take a look at the structure of an IPv6 Global Unicast address (GUA), a globally-routeable address:

IPv6 addresses, like IPv4 addresses, are Big Endian, that is, the most significant bits come first. That’s similar to decimal numbers, where thousands come before hundreds and so on. The most significant bits (the “high-order” bits) at the beginning of a GUA form the global routeing prefix. This is used by Internet Service Providers (ISPs) to route traffic through the global Internet.

The standard IPv6 global routeing prefix assigned to a single-site enterprise is a /48. RFC 3177 specified /48 as the standard prefix for all sites, even home users, but subsequently RFC 6177 recommended longer prefixes for small enterprises and home users. With IPv4 an enterprise may only be assigned a /24, if that. That means that ISPs have (very) roughly 16 million times the address space to work with (248 versus 224); in fact 48 bits provides about 280 trillion prefixes, or roughly 38,000 prefixes for every human on the planet.

It’s the structure at the local end of an IPv6 address that’s surprising for someone like me who has worked with IPv4 all through their career. The network prefix (what in IPv4 is called the subnet mask) is a fixed /64 in length, as opposed to the variable-length subnet masking (VLSM) of IPv4. That means that the interface ID (broadly the IPv6 equivalent of the host ID) is also 64 bits long. In other words there’s enough address space on each subnet for 264 hosts, i.e. 18446744073709551616, or enough M&Ms to fill the Great Lakes.

Now that’s a huge number. With current networking technology it’s difficult to imagine it being practical to have more than, say, 10000 hosts on a single subnet. That only requires 14 bits, so the other (most significant) 50 bits are effectively redundant, in other words 18446744073709535232 addresses or 99.99999999999991%. In practice there is likely to be parallel running of IPv4 and IPv6 on the same subnets for some time, and IPv4 subnets are typically much smaller, say, 256 hosts, so the redundancy will be greater still.

The consequence of this fixed /64 network prefix is that the subnet ID, which is the local part of the network prefix, is typically only sixteen bits. For IPv4, most enterprises use private RFC 1918 addressing combined with Network Address Translation. They often utilise the 10/8 private prefix subnetted down to a /24 for individual subnets, which also provides a 16-bit subnet addressing space. So in practice IPv6 doesn’t provide any additional subnet address space for many enterprises compared to IPv4.

Sixteen bits is still a large number, equivalent to 65536 addresses, and it would be very generous if enterprises used those sixteen bits as a completely flat address space. However they usually structure the local subnet space hierarchically, in order to aggregate routes efficiently and minimise internal routeing tables (just as the global routeing address space is structured). IPv6 does have the advantage for network administrators that addresses are represented in hex rather than IPv4’s decimal; each hex character represents four bits (a “nibble”) as opposed to the 8-bit components of an IPv4 address (an “octet”). This means that the subnet space can be divided conveniently into 4-bit chunks in a way that’s easy to read, and so less prone to error. It is generally recommended (by Coffeen 2014 for example) to follow this practice when designing your IPv6 subnetting structure.

Nevertheless creating a hierarchical structure in this way means that there will be inevitably some inefficiency in the use of the address space. For example, take a site that has 20 buildings with 20 subnets per building. Each nibble can encompass only 16 objects, so you would need to assign two nibbles to the site level of the hierarchy and two to the building level of the hierarchy, thus using up the available subnet space, with quite a lot of redundancy. This is almost a worse-case scenario, but it shows how the address space for local subnetting is not that generous, and looks positively stingy compared to the address space for interfaces.

In summary, IPv6 uses a roughly 48-bit address space for global routeing, That’s a huge amount of prefixes, and to be fair it does deliver the original goal of IPv6, which was to increase the global address space. However, the way that the local part of IPv6 addresses is structured means that there is no useable expansion of address space at the local end. That stuff about galaxies and light-years is (mostly) hype.

In the next post I’ll take a look at why IPv6 addresses are structured in this way.

That’s a lot of bits

© XKCD 2016

Up to now in my career I’ve been able to avoid IPv6, but I guess it’s time I faced the inevitable. IPv4 address exhaustion is upon us, and IPv6 adoption is creeping up: Google’s stats show a gradual increase in IPv6 connectivity, although IPv6 traffic is still very low. I live in Belgium, which at the time of writing has the highest level of IPv6 capability on the planet.

The original motivation for IPv6 was to expand the address space: in place of IPv4’s 32-bit addresses, IPv6 addresses are 128 bits, providing 2128 addresses, that’s 340282366920938463463374607431768211456, or about 340 trillion trillion trillion. That’s enough for an entire IPv4 Internet for every star in the universe, or an IPv6 address for every atom on the surface of the earth (and more than a hundred other planets) or if every address was a picometer (1 trillionth of a meter) then the whole address space would stretch 34 billion light years (while the furthest visible object in the universe is 13 billion light years away).

Let’s take a look at how IPv6 addresses are represented. Here’s an example of an IPv6 address:


The 128 bits are represented in hex, separated into eight groups of sixteen bits by colons. Sometimes the groups of sixteen bits are referred to as “quartets” (e.g. Writing IPv6 Addresses – Presented by Cricket Liu) and sometimes as “hextets” (e.g. Coffeen 2014, Chapter 2). I’ll follow Tom Coffeen’s practice and refer to them as hextets.

In the example address that I gave you’ll notice that there are only three hex digits in the second hextet. Leading zeroes within a hextet may be omitted, so that “:db8:” stands for “:0db8:”. A single hextet of all zeroes i.e. “:0000:” can be represented as a single zero “:0:”. A sequence of consecutive all-zero hextets can be represented as “::”, so that:


is represented as:


Since we know that an IPv6 address is 128 bits long, we can work out that there are five missing hextets that have been condensed down to a single “::”. At one point you could also condense a single hextet of all zeroes to “::” but RFC 5952 mandates the “:0:” format.

Things get more complicated if there are more than one sequence of consecutive all-zero hextets in the same IPv6 address. Here’s another address:


You can’t contract both sequences because you might end up with an ambiguous address: how many hextets would be represented by each “::”? Earlier RFCs didn’t document how you should handle this. RFC 5952 stipulated that you should condense the longest sequence of all-zero hextets, and if the two sequences are the same length then you should condense the first one, so the above example address would be condensed to:


RFC 5952 makes the rules on condensing all-zero hextets mandatory, and also specifies that systems should display IPv6 addresses with lower-case letters. It advises humans to follow the same rules.

IPv6 prefixes are represented in much the same way as IPv4 prefixes in CIDR notation, so that:


represents every address from:




In the next post I’ll take a look at the way IPv6 addresses are structured.