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11.3.5 IPv6 Facts

Internet connected devices have become pervasive, leading to a massive increase in the number of devices that connect to the internet. The 32-bit IPv4 address system has approximately 4.3 billion possible addresses and many of those are reserved.

Since every device that connects to the internet is assigned an IP address, the addresses available under the IPv4 addressing standard have been exhausted. To help remedy this situation, the Internet Engineering Task Force (IETF) developed the IPv6 standard in the mid 1990's.

This lesson covers the following topics:

IPv6 format
IPv6 components
IPv6 address types

IPv6 Format

The following list describes the format of an IPv6 address:

An IPv6 address is a 128-bit address made up of 8 16-bit blocks.
Each block is separated by a colon.
Each block is comprised of 4 hexadecimal values between 0000 and FFFF. Each block represents 16 bits of data (FFFF = 1111 1111 1111 1111).
There is approximately 2 ¹²⁸ (340 undecillion or 340 trillion trillion) available IPv6 addresses. The specific number of IPv6 addresses is 340,282,366,920,938,463,463,374,607,431,768,211,456

Because IPv6 addresses are so long, there are rules you can use to simplify an address. These rules are:

Leading zeros can be omitted in each section. For example, the quartet 0284 could also be written as 284.
An address with consecutive zeros can be expressed more concisely by substituting a double colon for the group of zeros. For example:
- FEC0:0000:0000:0000:78CD:1283:F398:23AB
- FEC0::78CD:1283:F398:23AB (concise form)
- Removing the zeros and simplifying the address is also known as address compression.
You can use address compression only once per address. For example, FEC2:0000:0000:0000:78CA:0000:0000:23AB can be abbreviated as:
- FEC2::78CA:0:0:23AB
  or
- FEC2:0:0:0:78CA::23AB
  but not
- FEC2::78CA::23AB

IPv6 Components

An IPv6 address is divided into two equal parts. The left side is the prefix and the right side is the interface ID. The prefix is comparable to the network ID in a IPv4 address and the interface ID is comparable to the host ID.

IPv6 Components Description

Prefix

The first 64-bits make up the prefix.

The prefix can be divided into various parts that identify things such as geographic region, the ISP, the network, and the subnet.
The first 48-bits of the address define the site prefix. The site prefix defines the location of the address and is assigned by the local ISP.
The next 16-bits make up the subnet ID. This defines the network the device is connected to.
CIDR notation can be used to indicate the prefix-length. As with an IPv4 address, this shows which portion of the address used for the prefix (network address).
- To indicate the prefix length, add a slash (/) followed by the prefix length.
- Full quartets with trailing 0s in the prefix address can be omitted (e.g., 2001:0DB8:4898:DAFC::/64).
Because addresses can be allocated based on physical location, the prefix generally identifies the location of the host. The prefix is often referred to as the global routing prefix.

Interface ID

The last 64-bits in the address is the interface ID. This is a unique identifier for each device, similar to a MAC address.

Addresses are assigned to interfaces (network connections), not to the host. Technically, the interface ID is not a host address, but is often referred to as the host address.
Interface IDs must be unique within a subnet, but the same interface ID can exist on multiple subnets.

To ensure that the interface ID is unique for every host on the network, IPv6 uses the Extended Unique Identifier 64 (EUI-64) format. Following are some details of the EUI-64 format:

Since each device is hard-wired with a unique 48-bit hardware address called the MAC address, the EUI-64 format uses it to generate a unique interface ID.
The EUI-64 format:
1. Converts the MAC address to binary.
2. Flips the 7 ^th bit from the left (from 1 to 0 or 0 to 1). This bit is called the universal/local (U/L) bit.
3. Inserts the 16-bit hexadecimal value FFFE in the middle.
For example, the address for a host with a MAC address of 20-0C-FB-BC-A0-07 would start with the following EUI-64 interface ID: 200C:FB FF:FE BC:A007

The interface ID can also be generated using a special algorithm that generates a completely randomized ID. This guarantees that each device will have a unique ID.

IPv6 Address Types

Every device on the network must be assigned an IP address. With IPv6, each device can have multiple addresses. The following table describes the different types of IPv6 addresses:

		Address Type Description
Unicast	Unicast addresses are assigned to a single interface for the purpose of allowing one host to send and receive data. Packets sent to a unicast address are delivered to the interface identified by that address. There are three types of unicast IPv6 addresses:
	Link-local	Link-local addresses (also known as local link addresses) are valid only on the current subnet. These addresses are similar to an APIPA IPv4 address. Details include: Link-local addresses have an FE80::/10 prefix. This includes any address beginning with FE8, FE9, FEA, or FEB. All nodes must have at least one link-local address, although each interface can have multiple addresses. Link-local addresses are used for automatic address configuration, for neighbor discovery, or for subnets that have no routers. Do not use link-local IPv6 addressing on routed networks. Routers do not forward packets destined for link-local addresses to other subnets.
	Unique local	Unique local addresses are private addresses used for communication within a site or between a limited number of sites. They are commonly used for network communications that do not cross a public network; they are the equivalent of private addressing in IPv4. Details include the following: Because unique local addresses are not registered with IANA, they cannot be used on a public network without address translation. Unique local addresses have an FC00::/7 prefix and include addresses beginning with FC or FD. Following the prefix, the next 40 bits are used for the Global ID. The Global ID is generated randomly, creating a high probability of uniqueness on the entire internet. Following the Global ID, the remaining 16 bits in the prefix are used for subnet information. Unique local addresses are likely to be globally unique, but they are not globally routable. Unique local addresses can be routed between sites by a local ISP. The process for designing a network addressing scheme when using unique local addresses is similar to that used for global unicast addresses. The key difference is how the prefix is defined. Because the address range is not registered, a global routing prefix does not have to be requested from an ISP. Instead, each organization defines its own prefix.
	Global unicast	Global unicast addresses are assigned to individual interfaces that are globally unique. All IPv6 addresses that aren't specifically reserved for other purposes are defined as global unicast addresses. The global routing prefix assigned to an organization by an ISP is typically 48 bits long (/48), but it could be as short as /32 or as long as /56, depending on the ISP. All subnet IDs within the same organization must begin with the same global routing prefix; they must be uniquely identified using the value in the subnet field. Using this addressing scheme allows organizations to define a large number (2 ¹⁶ ) of IPv6 subnets. When designing an IPv6 network, you can define the following separate IPv6 subnets: Network segments separated by routers VLANs Point-to-point WAN links
Multicast	Multicast addresses represent a dynamic group of hosts. Packets sent to a multicast address are sent to all interfaces identified by that address. If you use different multicast addresses for different functions, only the devices that need to participate in a particular function will respond to the multicast; devices that do not need to participate in the function will ignore the multicast. Details include: All multicast addresses have an FF00::/8 prefix. Multicast addresses that are restricted to the local link have an FF02::/16 prefix. Packets starting with FF02 are not forwarded by routers. Multicast addresses with an FF01::/16 prefix are restricted to a single node. The following are well-known multicast addresses: FF02::1 is for all nodes on the local link. This is the equivalent of the IPv4 subnet broadcast address. FF01::1 is for all interfaces on a node. FF02::2 is for all routers on the local link. FF01::2 is for all routers on node-local. FF02::1:2 is for all DHCP servers or DHCP relay agents on the local link. DHCP relay agents forward these packets to other subnets. There are no broadcast addresses in IPv6. IPv6 uses multicast addresses instead of broadcast addresses.
Anycast	The anycast address is a unicast address that is assigned to more than one interface, typically belonging to different hosts. An anycast packet is routed to the nearest interface having that address (based on routing protocol decisions). Details include: An anycast address is the same as a unicast address. Assigning the same unicast address to more than one interface makes it an anycast address. A anycast address can be a link-local, unique local, or global unicast address. When assigning an anycast address to an interface, it must be explicitly identified as an anycast address to distinguish it from a unicast address. You can use anycast addresses to locate the nearest server of a specific type. For example, the nearest DNS or network time server.
Loopback	The local loopback address for the local host is 0:0:0:0:0:0:0:1 (also identified as ::1 or ::1/128). The local loopback address is not assigned to an interface. It can verify that the TCP/IP protocol stack is properly installed on the host.