There are three different types of IPv6 addresses:

When reading an IPv6 address, the canonical form is represented as x:x:x:x:x:x:x:x, where each x represents a 16 bit hex value. An example is FEBC:A574:382B:23C1:AA49:4592:4EFE:9982.

Often, an address will have long substrings of all zeros. A :: (double colon) can be used to replace one substring per address. Also, up to three leading 0s per hex value can be omitted. For example, fe80::1 corresponds to the canonical form fe80:0000:0000:0000:0000:0000:0000:0001.

A third form is to write the last 32 bits using the well known IPv4 notation. For example, 2002::10.0.0.1 corresponds to the hexadecimal canonical representation 2002:0000:0000:0000:0000:0000:0a00:0001, which in turn is equivalent to 2002::a00:1.

To view a FreeBSD system's IPv6 address, use ifconfig(8):

# ifconfig

rl0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
         inet 10.0.0.10 netmask 0xffffff00 broadcast 10.0.0.255
         inet6 fe80::200:21ff:fe03:8e1%rl0 prefixlen 64 scopeid 0x1
         ether 00:00:21:03:08:e1
         media: Ethernet autoselect (100baseTX )
         status: active

In this example, fe80::200:21ff:fe03:8e1%rl0 is an auto-configured link-local address which was automatically generated from the MAC address.

Some IPv6 addresses are reserved. A summary of these reserved addresses is seen in Table 31.3, “Reserved IPv6 Addresses”:

For further information on the structure of IPv6 addresses, refer to RFC3513.

To configure a FreeBSD system as an IPv6 client, add these two lines to rc.conf:

ifconfig_em0_ipv6="inet6 accept_rtadv"
rtsold_enable="YES"

The first line enables the specified interface to receive router solicitation messages. The second line enables the router solicitation daemon, rtsol(8).

For FreeBSD 8.x, add a third line:

ipv6_enable="YES"

If the interface needs a statically assigned IPv6 address, add an entry to specify the static address and associated prefix length:

ifconfig_fxp0_ipv6="inet6 2001:db8:4672:6565:2026:5043:2d42:5344 prefixlen 64"

On a FreeBSD 8.x system, that line uses this format instead:

ipv6_ifconfig_fxp0="2001:db8:4672:6565:2026:5043:2d42:5344"

To assign a default router, specify its address:

ipv6_defaultrouter="2001:db8:4672:6565::1"

In order to connect to other IPv6 networks, one must have a provider or a tunnel that supports IPv6:

This section demonstrates how to take the directions from a tunnel provider and convert them into /etc/rc.conf settings that will persist through reboots.

The first /etc/rc.conf entry creates the generic tunneling interface gif0:

gif_interfaces="gif0"

Next, configure that interface with the IPv4 addresses of the local and remote endpoints. Replace MY_IPv4_ADDR and REMOTE_IPv4_ADDR with the actual IPv4 addresses:

gifconfig_gif0="MY_IPv4_ADDR REMOTE_IPv4_ADDR"

To apply the IPv6 address that has been assigned for use as the IPv6 tunnel endpoint, add this line, replacing MY_ASSIGNED_IPv6_TUNNEL_ENDPOINT_ADDR with the assigned address:

ifconfig_gif0_ipv6="inet6 MY_ASSIGNED_IPv6_TUNNEL_ENDPOINT_ADDR"

For FreeBSD 8.x, that line should instead use this format:

ipv6_ifconfig_gif0="MY_ASSIGNED_IPv6_TUNNEL_ENDPOINT_ADDR"

Then, set the default route for the other side of the IPv6 tunnel. Replace MY_IPv6_REMOTE_TUNNEL_ENDPOINT_ADDR with the default gateway address assigned by the provider:

ipv6_defaultrouter="MY_IPv6_REMOTE_TUNNEL_ENDPOINT_ADDR"

If the FreeBSD system will route IPv6 packets between the rest of the network and the world, enable the gateway using this line:

ipv6_gateway_enable="YES"

This section demonstrates how to setup rtadvd(8) to advertise the IPv6 default route.

To enable rtadvd(8), add the following to /etc/rc.conf:

rtadvd_enable="YES"

It is important to specify the interface on which to do IPv6 router solicitation. For example, to tell rtadvd(8) to use fxp0:

rtadvd_interfaces="fxp0"

Next, create the configuration file, /etc/rtadvd.conf as seen in this example:

fxp0:\
	:addrs#1:addr="2001:471:1f11:246::":prefixlen#64:tc=ether:

Replace fxp0 with the interface to be used and 2001:471:1f11:246:: with the prefix of the allocation.

For a dedicated /64 subnet, nothing else needs to be changed. Otherwise, change the prefixlen# to the correct value.

When IPv6 is enabled on a server, there may be a need to enable IPv4 mapped IPv6 address communication. This compatibility option allows for IPv4 addresses to be represented as IPv6 addresses. Permitting IPv6 applications to communicate with IPv4 and vice versa may be a security issue.

This option may not be required in most cases and is available only for compatibility. This option will allow IPv6-only applications to work with IPv4 in a dual stack environment. This is most useful for third party applications which may not support an IPv6-only environment. To enable this feature, add the following to /etc/rc.conf:

ipv6_ipv4mapping="YES"

Reviewing the information in RFC 3493, section 3.6 and 3.7 as well as RFC 4038 section 4.2 may be useful to some adminstrators.