IPv6 tutorial – Part 8: Special addresses

https://4sysops.com/archives/ipv6-tutorial-part-8-special-addresses/

The special IPv6 addresses discussed in this part of the IPv6 tutorial are the unspecified address, the loopback address, IPv4-mapped addresses, 6to4 addresses, multicast addresses, and the solicited-node address.

Last time, I talked about zone IDs and unique local IPv6 unicast addresses.

Today, I will introduce some special addresses.

In practice, usually only the networking guys really have to deal with these addresses.

But as a Windows admin, you should have at least heard of the terms.

Unspecified address

The IPv6 unspecified address is specified as “0:0:0:0:0:0:0:0” (or “::” if you have a sense for mathematical beauty).

I think, this naming convention is somewhat funny.It reminds me of the “unspeakable word.”

But there is also some sense in this because applications use the unspecified address as a variable for, well, an unspecified address.

I guess, you won’t see this address often, just like you didn’t have many encounters with the unspecified IPv4 address 0.0.0.0.

Note that you can’t assign this address to an interface, which also makes sense because then you would specify it, which is strictly forbidden.

Loopback address

Like the loopback address, IPv4-mapped addresses are only used for internal representation on a host.

They allow developers to use one API for both IPv4 and IPv6 calls.

The syntax of IPv4-mapped addresses looks like this: :FFFF:w.x.y.z, where “w.x.y.z” is the IPv4 address that is represented in the IPv6 address.

6to4 addresses

6to4 is a tunneling technique that allows two IPv6 cable systems to communicate over an IPv4 network (usually the Internet).

For this, at least one of the IPv6 systems needs an IPv4 address and a special IPv6 address, the so-called 6to4 address.

A computer that has a 6to4 address is called “6to4 host.” This transition technology will mostly likely be used by computers in IPv4 networks to reach IPv6-only hosts.

Essentially, the 6to4 address makes the 6to4 host appear on the “IPv6 landscape” even if it is not directly reachable through IPv6 by encapsulating IPv6 packets in IPv4 packets.

The IPv4 packets are converted to IPv6 packets and vice versa by a 6to4 gateway.

6to4 addresses have the prefix 2002::/16. The next 32 bits are the IPv4 address of the 6to4 host converted into hex.

Since a 6to4 address can only be created by using a unique public IPv4 address, the corresponding IPv6 address is also unique. The last 80 bits are for the local network and the host.

IPv6 multicast addresses

Like in IPv4, IPv6 multicast addresses are used to send the same data to multiple hosts simultaneously.

Since multicast-capable software, such as OS imaging tools, typically do all the IP configuration, Windows admins usually are not bothered with the networking details.

But if you ever stumble across an IPv6 address that begins with FF, you know what you are dealing with.

Solicited-node address

A special type of multicast address are the solicited-node addresses, which have the prefix FF02::1:FF00:0/104.

Solicited-node addresses are used for neighbor discovery, which I already discussed in my article about the new IPv6 features.

If a host needs to know the MAC address that belongs to a certain IP address, it won’t use an ARP broadcast like in IPv4.

Instead, it would use multicast to contact only those hosts that listen to the solicited-node addresses where the last 24 bits are identical to the ones in the IP address that has to be resolved.