Network Basics: IPv6 Special Address Identification

IPv6 handles special addresses differently from IPv4. With IPv4, you have seen that there may be special address groups that existed within the total IP address space. In addition to Class A, B, and C addressees, you also have the loopback address block (anything starting with 127) as well as multicast address space and Automatic Private IP Addressing (APIPA) addresses.

Within IPv6 are three main types of addresses, which are

  • Unicast: A single unique address for a network interface. There are several types of unicast addresses.

  • Multicast: A one-to-many relationship in which the IP address is actually a group address and many devices can belong to the group. Unlike IPv4, there is no such thing as a broadcast address that is processed by every device on a network.

    IPv6 relies on multicast and anycast addresses to be able to send data to more than a single unicast device on your network. Also, unlike IPv4, this new multicast address range is substantially larger than its predecessor, so it should be a long time before anyone runs into address limitations.

  • Anycast: A one-to-nearest relationship using unicast addresses. The difference between this and normal unicast traffic is that multiple devices use the same address, similar to a multicast group address. Communication might start with a unicast packet being sent to a multicast address in which the device that is closest to the sender would answer the request.

    Anycast addresses are a perfect solution for load-balancing problems (such as a web server farm) because multiple devices can use and share the same address, and only one device will respond to network requests from each client device. Because anycast addresses are allocated from the unicast pool, the address format for them is the same as that of the unicast addresses.

When configuring network interfaces for IPv6, a single network interface could have a number of addresses associated with it. This may be a mixture of these address types. Within the unicast type there are three main address groups:

  • Global addresses: These globally routable addresses are the addresses that are assigned by your ISP and include addresses in the 2000::/3 range. This range would include from 2000:: through to 39FF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF. Global addresses represent about one-eighth of all IPv6 addresses, and the numbers are handed out by the IANA, as they are with IPv4.

  • Reserved addresses: The Internet Engineering Task Force (IETF) reserved several addresses of the global address space to be implemented if it decides to introduce new features. This reserved space amounts to approximately 1 of every 256 IPv6 addresses.

  • Private addresses: All private addresses start with FE in the first two positions of this address, followed by another digit from 8 to F. This could also be written as FE80::/9. Like with IPv4, these addresses cannot be routed over the Internet. These private addresses fall into these major categories:

    • Site-local addresses are a section of the main private address space and include addresses in the FEC0::/10 block, which are all addresses that start with FE and have C, D, E, or F for their third character.

      These addresses are used to assigned IPv6 addresses to a site or organization (using an IPv6 DHCP server), and do not route outside that area or to the Internet. Internal routers forward data to these addresses on internal interfaces, but routers do not forward data to these out to the Internet.

      Because of a lack of definition around how site-local addresses were to be used, they were deprecated (marked and obsolete) in 2004, and replaced with a unique-local address in 2005.

    • Unique-local addresses are designed for an organization to assign internal addresses across its organization. These addresses are defined by the FC00::/7 address block, with the second half of the block, FD00::/8, allocated for use on networks and a description for FC00::/8 address block still in the works. As with the description of site-local addresses, these addresses are for internal-use only and are never routed out to the Internet.

    • Local-link addresses are similar to APIPA addresses, because they are self-generated and allow for IPv6 communications with other devices on that data link or network segment. These addresses all start with FE, and the third digit is 8, 9, A, or B, or FE80::/10. All IPv6 within a given data link that have local-link addresses can talk to each other. No routers, internal or external, forward traffic to or from these addresses.

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