Handbook of Local Area Networks, 1998 Edition:Advanced LAN Interconnectivity Issues and Solutions Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Address Prefix Allocation One of the goals of the IPv6 address format is to accommodate many different types of addresses. The beginning of an address contains a 3- to 10-bit format prefix defining the general address type; the remaining bits contain the actual host address, in a format specific to the indicated address type. Table 4-1-2 represents an address prefix allocation (from RFC 1884). TABLE 4-1-2. ADDRESS PREFIX ALLOCATION (FROM RFC 1884) Allocation Prefix (Binary) Fraction of Address Space Reserved 0000 0000 1/256 Unassigned 0000 0001 1/256 Reserved for NSAP     Allocation 0000 001 1/128 Reserved for IPX     Allocation 0000 010 1/128 Unassigned 0000 011 1/128 Unassigned0000 1 1/32 Unassigned 0001 1/16 Unassigned 001 1/8 Provider-Based     Unicast Address 010 1/8 Unassigned 011 1/8 Reserved for     Geographic-Based     Unicast Addresses 100 1/8 Unassigned 101 1/8 Unassigned 110 1/8 Unassigned 1110 1/16 Unassigned 1111 0 1/32 Unassigned 1111 10 1/64 Unassigned 1111 110 1/128 Unassigned 1111 1110 0 1/512 Link Local Use     Addresses 1111 1110 10 1/1024 Site Local Use     Addresses 1111 1110 11 1/1024 Multicast Addresses 1111 1111 1/256 The Provider-Based Unicast Address The provider-based unicast address is an IPv6 address that might be assigned by an Internet service provider (ISP) to a customer. Exhibit 4-1-2 shows a provider-based unicast address format. This type of address contains a number of subfields, including the following: •  Format prefix. This indicates the type of address as provider-based unicast. It is always 3 bits, coded “010.” •  Registry identifier. This identifies the Internet address registry from which the ISP obtains addresses. •  Provider identifier. This identifies the ISP. This field contains the address block assigned to the ISP by the address registry authority. •  Subscriber identifier. This identifies the ISP’s subscriber. This field contains the address assigned to this subscriber by the ISP. The provider ID and subscriber ID fields together are 56 bits in length. •  Intrasubscriber. This contains the portion of the address assigned and managed by the subscriber. Exhibit 4-1-2.  Provider-Based Unicast Address Format IPv4-Compatible Addresses Another particularly important address type is the one that indicates an IPv4 address. With more than 16 million hosts using 32-bit addresses, the public Internet must continue to accommodate IPv4 addresses even as it slowly migrates to IPv6 addressing, IPv4 addresses are carried in a 128-bit IPv6 address that begins with 80 zeros (0:0:0:0:0). The next 16-bit block contains the compatibility bits, which indicate the way in which the host/router handles IPv4 and IPv6 addresses. If the device can handle either IPv4 or IPv6 addresses, the compatibility bits are all set to zero(0) and this is termed an “IPv4-compatible IPv6 address”; if the address represents an IPv4-only node, the compatibility bits are all set to one (0xFFFF) and the address is termed an “IPv4-mapped IPv6 address.” The final 32 bits contain a 32-bit IPv4 address in dotted decimal form. Multicast Addresses IPv6 multicast addresses provide an identifier for a group of nodes. A node may belong to any number of multicast groups. Multicast addresses may not be used as a source address in IPv6 packets or appear in any routing. All multicast addresses, as shown in Exhibit 4-1-3, begin with 8 ones (0xFF). The next 4 bits are a set of flag bits (flgs); the 3 high-order bits are set to zero; and the fourth bit (T-bit) indicates a permanently assigned (“well-known”) multicast address (T=0) or a nonpermanently assigned (“transient”) multicast address (T=1). The next 4 bits indicate the scope of the address, or the part of the network for which this multicast address is relevant; options include node-local (0x1), link-local (0x2), site-local (0x5), organization-local (0x8), or global (0xE). Exhibit 4-1-3.  Multicast Address Format The remaining 112 bits are the group identifier, which identifies the multicast group, either permanent or transient, within the given scope. The interpretation of a permanently assigned multicast address is independent of the scope value. For example, if the World Wide Web (WWW) server group is assigned a permanent multicast address with a group identifier of 0x77, then: •  FF01:0:0:0:0:0:0:77 would refer to all WWW servers on the same node as the sender. •  FF02:0:0:0:0:0:0:77 would refer to all WWW servers on the same link as the sender. •  FF05:0:0:0:0:0:0:77 would refer to all WWW servers at the same site as the sender. •  FF0E:0:0:0:0:0:0:77 would refer to all WWW servers in the Internet. Finally, a number of well-known multicast addresses are predefined, including: •  Reserved multicast addresses. These are reserved and are never assigned to any multicast group. These addresses have the form FF0x:0:0:0:0:0:0:0, where x is any hexadecimal digit. •  All nodes’ addresses. These identify the group of all IPv6 nodes within the given scope. These addresses are of the form FF0t:0:0:0:0:0:0:1, where t =1 (node-local) or 2 (link-local). •  All routers’ addresses. These identify the group of all IPv6 routers within the given scope. These addresses are of the form FF0t: 0:0:0:0:0:0:2, where t =1 (node-local) or 2 (link-local). •  The dynamic host configuration protocol (DHCP) server/relay-agent address. This identifies the group of all IPv6 DHCP servers and relay agents with the link-local scope; this address is FF02:0:0:0:0:0:0:C. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.