A quick discussion about IPv6 IP addressing

 Today I am going to talk about the IPv6 IP addressing space and the subnetting in details. As per the less availability of the IPv4 IP addresses all Enterprises now moving to IPv6 ip addressing and which has almost 4 times IP available in the pool. 

Some of you already knew about the IPv6 addressing and some of you are just starting using the IPv6 ip addressing.

Let's talk about the IPv6 ip addressing to understand how we can recognize the IP in the network and further how we can use that IP addresses for subnetting purposes. 

I hope this article will help at some extend to understand the IPv6 ip addressing and if there is any question about the IPv6 addressing please comment so that we can further dig-out the questions and will reply as per the requests.

The '/' indicates the bits which are fixed for your sub network. Fixed means that you do not have any control on these bits, you must not change them (I like to call them "don't touch only watch bits"). IPv6 is 128 bits long and your network/organization is assigned "128 minus /prefix-length Value" and these are the bits given under your control. 

You can do your subnetting with these bits and assign addresses to your computer/car/mobile/etc.

Fig 1.1- IPv6 Addressing Formats

For example, assume that the prefix '2001:db8:1234::/48' is assigned to your network by your service provider (or by RIPE, ARIN, etc.). In this case, /48 indicates that starting from the left-most bit up to and including the 48th bit, these bits are fixed and must not be changed (don't touch only watch). And the rest of the bits which have the length of 128-48=80 bits are given to your control.

So, the importance of '/' prefix-length comes from the point that it indicates the boundary of your network. In other words, from which number your assigned network starts and at which number it 

IPv6 has three types of addresses, which can be categorized by type and scope:

  1. Unicast addresses. A packet is delivered to one interface.
  2. Multicast addresses. A packet is delivered to multiple interfaces.
  3. Any-cast addresses. A packet is delivered to the nearest of multiple interfaces (in terms of routing distance).

IPv6 does not use broadcast messages.
Unicast and any-cast addresses in IPv6 have the following scopes (for multicast addresses, the scope is built into the address structure):

  1. Link-local. The scope is the local link (nodes on the same subnet).
  2. Site-local. The scope is the organization (private site addressing).
  3. Global. The scope is global (IPv6 Internet addresses).

Fig 1.2- IPv6 Addressing (cont.)

The leftmost three fields (48 bits) contain the site prefix. The prefix describes the public topology that is usually allocated to your site by an ISP or Regional Internet Registry (RIR). 

The next field is the 16-bit subnet ID, which you (or another administrator) allocate for your site. The subnet ID describes the private topology, also known as the site topology, because it is internal to your site.

The rightmost four fields (64 bits) contain the interface ID, also referred to as a token. The interface ID is either automatically configured from the interface's MAC address or manually configured in EUI-64 format.


This example shows all 128 bits of an IPv6 address. The first 48 bits, 2001:0db8:3c4d, contain the site prefix, representing the public topology. The next 16 bits, 0015, contain the subnet ID, representing the private topology for the site. The lower order, rightmost 64 bits, 0000:0000:1a2f:1a2b, contain the interface ID

Prefixes in IPv6
The leftmost fields of the IPv6 address contain the prefix, which is used for routing IPv6 packets. IPv6 prefixes have the following format:

prefix/length in bits
Prefix length is stated in classless inter-domain routing (CIDR) notation. CIDR notation is a slash at the end of the address that is followed by the prefix length in bits. For information on CIDR format IP addresses. The site prefix of an IPv6 address occupies up to 48 of the leftmost bits of the IPv6 address. For example, the site prefix of the IPv6 address 2001:db8:3c4d:0015:0000:0000:1a2f:1a2b/48  is contained in the leftmost 48 bits, 2001:db8:3c4d. You use the following representation, with zeros compressed, to represent this prefix:


You can also specify a subnet prefix, which defines the internal topology of the network to a router. The example IPv6 address has the following subnet prefix.


The subnet prefix always contains 64 bits. These bits include 48 bits for the site prefix, in addition to 16 bits for the subnet ID.

The following prefixes have been reserved for special use:
Indicates that a 6to4 routing prefix follows.
Indicates that a link-local address follows.
Indicates that a multicast address follows.

Fig 1.3- IPv6 Addressing

Multicast solicited node address
The IPv6 multicast solicited node address is used for efficient address resolution. The IPv4 ARP Request frame is sent to the MAC-level broadcast, which disturbs all nodes on the network segment. 

The multicast solicited node address combines the prefix FF02::1:FF00:0/104 with the last 24 bits of the IPv6 address being resolved. 

IPv6 uses the solicited node multicast address for the Neighbor Solicitation message (the IPv6 equivalent to the ARP Request frame) that resolves an IPv6 address to its link-layer address, disturbing few nodes during the address resolution process.

Any-cast IPv6 Addresses
Any-cast IPv6 addresses are similar to but more efficient than the any-cast addresses in IPv4, which are used primarily by large ISPs. Any-cast addresses use the unicast address space but function differently from other unicast addresses. IPv6 uses any-cast addresses to identify multiple interfaces. 

IPv6 delivers packets addressed to an any-cast address to the nearest interface that the address identifies. In contrast to a multicast address, where delivery is from one to many, an any-cast address delivery is from one to one-of-many. 

Currently, any-cast addresses are assigned only to routers and are used only as destination addresses.