In order to make writing addresses containing zero bits easier a special syntax is available to compress the zeros. The use of "::" indicates multiple groups of bits of zeros. The "::" can only appear once in an address. For example the following addresses: CA a unicast address FF a multicast address the loopback address the unspecified addresses may be represented as: CA a unicast address FF a multicast address the loopback address :: the unspecified addresses 3.
An alternative form that is sometimes more convenient when dealing with a mixed environment of IPv4 and IPv6 nodes is x:x:x:x:x:x:d. Examples: The variable-length field comprising these leading bits is called the Format Prefix FP. See section 2. This allocation supports the direct allocation of aggregation addresses, local use addresses, and multicast addresses.
The remainder of the address space is unassigned for future use. This can be used for expansion of existing use e. Fifteen percent of the address space is initially allocated. Unicast addresses are distinguished from multicast addresses by the value of the high-order octet of the addresses: a value of FF identifies an address as a multicast address; any other value identifies an address as a unicast address.
Anycast addresses are taken from the unicast address space, and are not syntactically distinguishable from unicast addresses. There are several forms of unicast address assignment in IPv6, including the global aggregatable global unicast address, the NSAP address, the IPX hierarchical address, the site-local address, the link-local address, and the IPv4-capable host address.
Additional address types can be defined in the future. IPv6 nodes may have considerable or little knowledge of the internal structure of the IPv6 address, depending on the role the node plays for instance, host versus router.
Though a very simple router may have no knowledge of the internal structure of IPv6 unicast addresses, routers will more generally have knowledge of one or more of the hierarchical boundaries for the operation of routing protocols. The known boundaries will differ from router to router, depending on what positions the router holds in the routing hierarchy. They are required to be unique on that link. They may also be unique over a broader scope. In many cases an interface's identifier will be the same as that interface's link- layer address.
The same interface identifier may be used on multiple interfaces on a single node. Note that the use of the same interface identifier on multiple interfaces of a single node does not affect the interface identifier's global uniqueness or each IPv6 addresses global uniqueness created using that interface identifier. In a number of the format prefixes see section 2. EUI based Interface identifiers may have global scope when a global token is available e. The "u" bit is set to one 1 to indicate global scope, and it is set to zero 0 to indicate local scope.
The motivation for inverting the "u" bit when forming the interface identifier is to make it easy for system administrators to hand configure local scope identifiers when hardware tokens are not available. This is expected to be case for serial links, tunnel end- points, etc. The alternative would have been for these to be of the form , , etc.
It must never be assigned to any node. It indicates the absence of an address. One example of its use is in the Source Address field of any IPv6 packets sent by an initializing host before it has learned its own address. It may be used by a node to send an IPv6 packet to itself. It may never be assigned to any physical interface. It may be thought of as being associated with a virtual interface e. The loopback address must not be used as the source address in IPv6 packets that are sent outside of a single node.
An IPv6 packet with a destination address of loopback must never be sent outside of a single node and must never be forwarded by an IPv6 router.
IPv6 nodes that utilize this technique are assigned special IPv6 unicast addresses that carry an IPv4 address in the low- order bits. This document recommends that network implementors who have planned or deployed an OSI NSAP addressing plan, and who wish to deploy or transition to IPv6, should redesign a native IPv6 addressing plan to meet their needs.
These mechanisms are the ones that must be used if such support is required. This document also defines a mapping of IPv6 addresses within the OSI address format, should this be required. This address format is designed to support both the current provider based aggregation and a new type of aggregation called exchanges. The combination will allow efficient routing aggregation for both sites which connect directly to providers and who connect to exchanges.
Sites will have the choice to connect to either type of aggregation point. These are Link-Local and Site-Local. The Link-Local is for use on a single link and the Site-Local is for use in a single site. Routers must not forward any packets with site-local source or destination addresses outside of the site. Anycast addresses are allocated from the unicast address space, using any of the defined unicast address formats.
Thus, anycast addresses are syntactically indistinguishable from unicast addresses. When a unicast address is assigned to more than one interface, thus turning it into an anycast address, the nodes to which the address is assigned must be explicitly configured to know that it is an anycast address.
For any assigned anycast address, there is a longest address prefix P that identifies the topological region in which all interfaces belonging to that anycast address reside.
Within the region identified by P, each member of the anycast set must be advertised as a separate entry in the routing system commonly referred to as a "host route" ; outside the region identified by P, the anycast address may be aggregated into the routing advertisement for prefix P.
Note that in, the worst case, the prefix P of an anycast set may be the null prefix, i. Therefore, it is expected that support for global anycast sets may be unavailable or very restricted.
One expected use of anycast addresses is to identify the set of routers belonging to an organization providing internet service. Such addresses could be used as intermediate addresses in an IPv6 Routing header, to cause a packet to be delivered via a particular aggregation or sequence of aggregations.
Some other possible uses are to identify the set of routers attached to a particular subnet, or the set of routers providing entry into a particular routing domain. There is little experience with widespread, arbitrary use of internet anycast addresses, and some known complications and hazards when using them in their full generality [ ANYCST ]. Until more experience has been gained and solutions agreed upon for those problems, the following restrictions are imposed on IPv6 anycast addresses: o An anycast address must not be used as the source address of an IPv6 packet.
This anycast address is syntactically the same as a unicast address for an interface on the link with the interface identifier set to zero. Packets sent to the Subnet-Router anycast address will be delivered to one router on the subnet.
All routers are required to support the Subnet-Router anycast addresses for the subnets which they have interfaces. For example when a mobile host needs to communicate with one of the mobile agents on its "home" subnet. A node may belong to any number of multicast groups. User icon An illustration of a person's head and chest.
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