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Basic IPv6 concepts

    Basic concept

    IPv6 is a completely separate protocol from IPv4! It means that the configuration and operation of IPv6 have nothing to do with those of IPv4 at all. They will run independently just like IPv4 and IPX or AppleTalk.

    Format of IPv6 address

    • An IPv6 address contains 128 bits and is written in 8 groups of 16 bits like abcd:0000:1111:2222:1357:2468:5555:6789. Each group contains 4 hexadecimal digits (so, 16 bits).
    • Many IPv6 addresses contain many consecutive groups of 0000 such as abcd:0000:0000:0000:0000:0000:0000:1234. To save typing, such consecutive groups of zero can be written as ::, so the address above can be written as abcd::1234. Or working from the reverse direction, because there are eight groups in an IPv6 address but only two groups were given, so :: is representing the six groups that are all zero.

    Different types of IPv6 addresses

    • Just like IPv4, there are global IPv6 addresses that can be obtained from ISP. They begin with 2????: or 3???:.
    • Just like private IPv4 addresses like 192.168.?.?, in IPv6 we use fec?:, fed?:, fee?:, fef?:. These are called site local addresses as they can only be used privately on a site. However, IPv6 provides a better alternative called ‘unique local address’ in the range of fd??. To use it you will first generate a random number and append it to fd to form a random prefix, so that it will not collide (in practice) with any other range used by other private networks.
    • Just like in IPv4, there is ::1 (i.e., 0000:0000:0000:0000:0000:0000:0000:0001) in IPv6.
    • Just like 169.254.?.? in IPv4, there are fe8?:, fe9?:, fea?:, feb?: address ranges in IPv6. They are called link local addresses as they can only be used privately on a link (a broadcast domain such as a switch or a set of interconnected switches). They are mainly used for network discovery (see below).

    Netmask and subnetting

    The host bits in IPv6 take up at least the lower 64 bits (and maybe more), so the netmask is always <= 64 bits. The only exception is the loopback address ::1 which has a netmask of /128. For a LAN, usually just use /64 as the netmask. The ISP may assign a /64, /56 or /48 address block to a site.


    • Stateless auto configuration. In IPv6 it is fine to let a host configure its own address by getting the prefix (the network ID) from the router and then using its own MAC address as the host ID (with some encoding). Therefore, you only need to configure the router with static IPv6 address (so it knows the prefix). How can the host find the router? The host can use multicast to discover the router (part of the IPv6 Network Discovery Protocol) so you don’t need to manually configure the default gateway of the host. The idea is that it can detect the running router automatically and perform fail-over as needed.
    • DHCPv6. It is very much like DHCP for IPv4. Use it if you need centralized control.


    • Domain name to address. You use an AAAA record to map a domain name to an IPv6 address. You can use either IPv4 or IPv6 to query the server and it can return A or AAAA records in either case (depending what record type you are searching for).
    • NS, MX, etc. You can just put a domain name there and the name server will try to find its A and AAAA records and return them in the additional section.
    • PTR. The reverse domain name is in the form of a.b.c.d.?.?.?.?.?.?.?.IPv6.arpa where a, b, c and d represent a hexadecimal digit (the address is like ??:??:??:…:dc:ba).

    Binding with application protocols

    Most higher level protocols already support IPv4 and IPv6 simultaneously (e.g., ping/ping6, OpenSSH, Chrome, Firefox, Apache). To use an IPv6 address directly, you may need to put it in square brackets, e.g., http://[2001::1] in a browser otherwise the colon may look like starting a port. If you use a domain name and already has an AAAA record, the OS will return both the IPv4 and IPv6 to the application. According to the standard, actually global IPv6 are listed before (preferred) public IPv4.

    In Linux, you can configure a static IPv6 address just like IPv4 in the interfaces file:

    iface eth0 inet6 static
    address 2001::1
    netmask 64

    Checking the IPv6 addresses can be done with:

    # ip addr show


    Reference: Basic IPv6 concepts from our JCG partner Kent Tong at the Kent Tong’s personal thoughts on information technology blog.

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