![]() |
|||
![]()
|
![]() |
![]() Click Here! |
![]() |
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).
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:
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).
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:
Finally, a number of well-known multicast addresses are predefined, including:
|
![]() |
|
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. |