IPv6

Testing your IPv6 connection:

https://ifconfig.co/

Including CLI-support

https://ready.chair6.net/

Nice as simple interface

https://test-ipv6.com/

Testing website security and connectivity

https://internet.nl

Below seems to be broken December 2025

https://ipv6-test.com/

Check your IPv6 configuration

http://ipv6-test.com/validate.php

Check IPv6 support for websites

Address Assignment

The IPv6 address assignment is handled by IANA. Most important prefixes are:

2000::/3	Global Unicast	Global scope
FC00::/7	Unique Local Unicast	Local Unicast. For intranets, not routed, like IPv4 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Currently FC00::/8 is undefined, only FD00::/8 is used
FE80::/10	Link Local Unicast	Link Identification, addresses can be based on the interface MAC address
FF00::/8	Multicast

Local Unicast (ULA)

Users were advised to generate a 40 bits Global ID for their intranets but it is unclear if there a valid use case for it[[1]]. It is advised to calculate the Global Id using a specified procedure to maximize the probability of an unique address.

Local Unicast Address-format:

Prefix	L	Global ID	Subnet ID	Interface ID
7 bits FC00::/7	1 bit Locally assigned	40 bits User defined	16 bits Local subnet	64 bits Locally assigned or by dhcp
1111110	1

The Interface ID is in the so-called ‘Modified EUI-64 Format’. How to go from a MAC address to Modified EUI-64 format is described in appendix A of [href="http://tools.ietf.org/html/rfc4291 RFC 4291]. When using ULA prefixes you must have Routing Advertising (radvd) to tell the network it should configure addresses and routes for it

Route Advertisement

Provides information for ‘Stateless Autoconfiguration’ to the local nodes.

IPv6 systems can send a Route Solicitation (RS) messages. These are answered by a Route Advertisement (RA) message that tells with prefixes exist and, when needed where the gateway for that prefix is. The client then can configure IPv6 addresses and routes.

On linux systems there is the radvd deamon to provide RA.

To advertise there is a ULA prefix in the network put this in /etc/radvd.conf

interface eth0
{
    AdvSendAdvert on;
    AdvDefaultLifetime 0;
    prefix fdxx:xx:xx::/48
    {
    };
};

Disable IPv6 on a system

At the end of /etc/sysctl.conf add:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

sysctl -p

Force reload of the kernel parameters as in /etc/sysctl.conf

cat /proc/sys/net/ipv6/conf/all/disable_ipv6

Check if IPv6 has been disabled:

0 => IPv6 is not disabled

1 => IPv6 is disabled

Check IPv6 autoconfiguration

cat /proc/sys/net/ipv6/conf/eth0/autoconf

0 => autoconf is disabled

1 => autoconf is enabled

Misc commands

/sbin/ip -6 addr del <ipv6address>/<prefixlength> dev <interface>

Remove ipv6 addres from an interface