An anycast address identifies one or more interfaces. Notice that the term device isn’t used since a device can have more than one interface. Sometimes people use the term node to designate an interface on a device. Basically, an anycast is a hybrid of a unicast and multicast address.
For example, a multicast address that begins with FF02::/16 is a permanent link address, whereas an address of FF15::/16 is a temporary address for a site.
Unicast IPv6 Addresses
The following types of addresses are unicast IPv6 addresses:
IPv6 global addresses are equivalent to public IPv4 addresses. They are globally routable and reachable on the IPv6 Internet. Global unicast addresses are designed to be aggregated or summarized for an efficient routing infrastructure. Unlike the current IPv4-based Internet, which is a mixture of both flat and hierarchical routing, the IPv6-based Internet has been designed from its foundation to support efficient, hierarchical addressing and routing. The scope of a global address is the entire IPv6 Internet. RFC 4291 defines global addresses as all addresses that are not the unspecified, loopback, link-local unicast, or multicast addresses. However, Figure shows the structure of global unicast addresses defined in RFC 3587 that are currently being used on the IPv6 Internet.
The structure of global unicast addresses defined in RFC 3587 The fields in the global unicast address are described in the following list:
Local-use unicast addresses do not have a global scope and can be reused. There are two types of local-use unicast addresses:
Link-local addresses are a new concept in IPv6. These kinds of addresses have a smaller scope as to how far they can travel: just the local link (the data link layer link). Routers will process packets destined to a link-local address, but they will not forward them to other links. Their most common use is for a device to acquire unicast site-local or global unicast addressing information, discovering the default gateway, and discovering other layer 2 neighbors on the segment. IPv6 link-local addresses, identified by the initial 10 bits being set to 1111 1110 10 and the next 54 bits set to 0, are used by nodes when communicating with neighboring nodes on the same link. For example, on a single-link IPv6 network with no router, link-local addresses are used to communicate between hosts on the link. IPv6 link-local addresses are similar to IPv4 link-local addresses defined in RFC 3927 that use the 169.254.0.0/16 prefix. The use of IPv4 link-local addresses is known as Automatic Private IP Addressing (APIPA) in Windows Vista, Windows Server 2008, Windows Server 2003, and Windows XP. The scope of a link local address is the local link. A link-local address is required for some Neighbor Discovery processes and is always automatically configured, even in the absence of all other unicast addresses. Link-local addresses always begin with FE80. With the 64-bit interface identifier, the prefix for link-local addresses is always FE80::/64.
An IPv6 router never forwards link-local traffic beyond the link.
represent a particular site or company. These addresses can be used within a company without having to waste any public IP addresses—not that this is a concern, given the large number of addresses available in IPv6. However, by using private addresses, you can easily control who is allowed to leave your network and get returning traffic back by setting up address translation policies for IPv6. Site-local addresses, identified by setting the first 10 bits to 1111 1110 11, are equivalent to the IPv4 private address space (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16). For example, private intranets that do not have a direct, routed connection to the IPv6 Internet can use site local addresses without conflicting with global addresses. Site-local addresses are not reachable from other sites, and routers must not forward site-local traffic outside the site. Site-local addresses can be used in addition to global addresses. The scope of a site-local address is the site. Unlike link-local addresses, site-local addresses are not automatically configured and must be assigned either through stateless or stateful address autoconfiguration. The first 10 bits are always fixed for site-local addresses, beginning with FEC0::/10. After the 10 fixed bits is a 54-bit Subnet ID field that provides 54 bits with which you can create subnets within your organization. You can have a flat subnet structure, or you can divide the high order bits of the Subnet ID field to create a hierarchical and summarize able routing infrastructure. After the Subnet ID field is a 64-bit Interface ID field that identifies a specific interface on a subnet. Site-local addresses have been formally deprecated in RFC 3879 for future IPv6 implementations. However, existing implementations of IPv6 can continue to use site-local addresses.
Zone IDs for Local-Use Addresses
Unlike global addresses, local-use addresses (link-local and site-local addresses) can be reused. Link-local addresses are reused on each link. Site-local addresses can be reused within each site of an organization. Because of this address reuse capability, link-local and site-local addresses are ambiguous. To specify the link on which the destination is located or the site within which the destination is located, an additional identifier is needed. This additional identifier is a zone identifier (ID), also known as a scope ID, which identifies a connected portion of a network that has a specified scope. The syntax specified in RFC 4007 for identifying the zone associated with a local-use address is Address%zone ID, in which Address is a local-use unicast IPv6 address and zone ID is an integer value representing the zone. The values of the zone ID are defined relative to the sending host. Therefore, different hosts might determine different zone ID values for the same physical zone. For example, Host A might choose 3 to represent the zone of an attached link and Host B might choose 4 to represent the same link.
Unique Local Addresses
Site-local addresses provide a private addressing alternative to global addresses for intranet traffic. However, because the site-local address prefix can be reused to address multiple sites within an organization, a site-local address prefix can be duplicated. The ambiguity of site local addresses in an organization adds complexity and difficulty for applications, routers, and network managers.
To replace site-local addresses with a new type of address that is private to an organization yet unique across all the sites of the organization, RFC 4193 defines unique local IPv6 unicast addresses. The first 7 bits have the fixed binary value of 1111110. All local addresses have the address prefix FC00::/7. The Local (L) flag is set 1 to indicate that the prefix is locally assigned. The L flag value set to 0 is not defined in RFC 3879. Therefore, unique local addresses within an organization with the L flag set to 1 have the address prefix of FD00::/8. The Global ID identifies a specific site within an organization and is set to a randomly derived 40-bit value. By deriving a random value for the Global ID, an organization can have statistically unique 48-bit prefixes assigned to their sites. Additionally, two organizations that use unique local addresses that merge have a low probability of duplicating a 48-bit unique local address prefix, minimizing site renumbering. Unlike the Global Routing Prefix in global addresses, the Global IDs in unique local address prefixes are not designed to be summarized. Unique local addresses have a global scope, but their reach ability is defined by routing topology and filtering policies at Internet boundaries. Organizations will not advertise their unique local address prefixes outside of their organizations or create DNS entries with unique local addresses in the Internet DNS. Organizations can easily create filtering policies at their Internet boundaries to prevent all unique local-addressed traffic from being forwarded. Because they have a global scope, unique local addresses do not need a zone ID. The global address and unique local address share the same structure beyond the first 48 bits of the address. In both addresses, the 16-bit Subnet ID field identifies a subnet within an organization. Because of this, you can create a subnetted routing infrastructure that is used for both local and global addresses. For example, a specific subnet of your organization can be assigned both the global prefix 2001:DB8:4D1C:221A::/64 and the local prefix FD0E:2D:BA9:221A::/64, where the subnet is identified for both types of prefixes by the Subnet ID value of 221A. Although the subnet identifier is the same for both prefixes, routes for both prefixes must still be propagated throughout the routing infrastructure so that addresses based on both prefixes are reachable.
Summary tables of IPv6 Addresses
These are assigned by the IANA and used on public networks. They are equivalent to IPv4 global (sometimes called public) addresses. ISPs summarize these to provide scalability in the Internet.
Reserved addresses are used for specific types of anycast as well as for future use. Currently about 1/256th of the IPv6 address space is reserved.
Like IPv4, IPv6 supports private addressing, which is used by devices that don’t need to access a public network. The first two digits are FE, and the third digit can range from 8 to F.
Like the 127.0.0.1 address in IPv4, 0:0:0:0:0:0:0:1, or ::1, is used for local testing functions; unlike IPv4, which dedicates a complete A class block of addresses for local testing, only one is used in IPv6.
0.0.0.0 in IPv4 means “unknown” address. In IPv6, this is represented by 0:0:0:0:0:0:0:0, or ::, and is typically used in the source address field of the packet when an interface doesn’t have an address and is trying to acquire one dynamically.
In our next article we will discus about special IPv6 address, IPv4 address and their equivalents IPv6 address. And then we learn how to assign these addresses to host, router and other devices.
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