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IPv6 and FI

The Faculty of Informatics has been assigned a network prefix by the CESNET connection provider 2001:0718:0801:0200::/56 .

If you want to set up IPv6 on your machine, contact the administrator at . The preferred option is to send us a DUID, or it is possible to set the address statically. More about both options below. If you configure an IPv6 address on a machine you manage, we will also enter this address into DNS (AAAA record).

State configuration

For interest, we will describe the convention we follow when assigning addresses. It is not necessary to understand it, if you wish to set an IPv6 address, contact us and we will tell you the address.

We assign a public IPv6 address to machines with a public IPv4 address that logically corresponds to an IPv4 address as follows:

  • Take, for example, a machine with an address in the net .
  • The first 16 bits ( 147.251 ) belongs to the MU network, by adding another 8 bits ( 42 ) creates a (sub) network address, ie . 8 bits left ( 106 ) identifies the machine within the network.
  • 42 is hexadecimal 2a , 106 Yippee 6a .
  • So let's take our 56 bits ( 2001:0718:0801:02 ), we will add to them 2a and thus a (sub) network is created 2001:718:801:22a::/64 which corresponds to the (sub) network .
  • We will add 6a and we get the address 2001:718:801:22a::6a

The gateway is usually at 1 (excluding network , respectively 2001:718:801:230::1/64 where 1 is for historical reasons Aisa; gateway is at , respectively 2001:718:801:230::e ).


The preferred method is stateful configuration (DHCPv6). All you need to know is the DUID (DHCP Unique Identifier) of the device. However, there is usually a problem with detecting / setting up a DUID at the client. It depends on the DHCP client and it is not always easy. We usually detect it by scanning network traffic with a tool tcpdump , which is a procedure that works regardless of the DHCP client on the machine. For example, you can capture it like this (remove those from the command) -e option_name that your TShark does not know):

tshark -i $IFACE -Y 'udp port 547' -T fields -E header=y \
    -E separator=/t -e eth.src -e dhcpv6.partial_name_preceded_by_fqdn \
    -e dhcpv6.client_fqdn -e ipv6.src -e dhcpv6.duid.bytes

In addition, it needs (for the functionality of router advertisements ) to use the machine LLA based on MAC address and not randomly generated ( privacy extensions ). The DHCP client also must not request so-called prefix delegation; this behavior is known systemd-networkd , to which must be added to the configuration:


Static configuration

We often use manual (static) configuration on servers. If we want to set up IPv6, for example, machines with an IPv4 address , we will set a logically corresponding IPv6 address and the configuration will look like this:

Adresa: 2001:718:801:22a::6a/64
Gateway: 2001:718:801:22a::1

Stateless configuration


When stateless autoconfiguration ( SLAAC ) the client chooses the address himself. This method is used in Eduroam and wlan_fi wireless networks and also in some virtual machines in the faculty cloud infrastructure Stratus.FI (Virtual machines are usually assigned a NATed private IPv4 address and a public IPv6 address).