scussed in Chapter 9, in the section "IPsec VPNs"). The encryp- tion extension known as the Encapsulating Security Payload (ESP) service renders the packet's payload data illegible unless the recipient has the proper key to unscramble the data. Administrators can choose to encrypt only the transport and data payload of a packet or the entire packet, including headers and extensions. The architecture for IPv6 is defined in RFC 3513, "IP Version 6 Addressing Architecture," which is available at www.ietf.org/rfc/rfc3513.txt and provides com- plete information on IPv6 addressing, rules, and exceptions. IPv6 addressing rules are covered by multiple RFCs. The Transition from IPv4 to IPv6 There are several techniques for enabling IPv4-to-IPv6 interworking and coexistence: IPv4-to-IPv6 translation-IPv4-to-IPv6 translation, shown in Figure 8.15, involves a network address translator, or a protocol translator. It employs stateless IP and ICMP translations. It is a bidirectional translation algorithm for IPv4, IPv6, ICMPv4, and ICMPv6 packet headers. Internet Addressing and Address Resolution IPv4 Packets IPv6 Packets Network Address Network Network Translator Figure 8.15 IPv4-to-IPv6 translation Dual Stack-With Dual Stack, which is already widely deployed, IPv4 and IPv6 exist in the same host (see Figure 8.16). New Dual Stack IPv6 devices will be backward capable with IPv4, and IPv4 devices can be programmed with both IPv4 and IPv6 stacks to process respective packets appropriately. 6to4-The third alternative involves IPv6 tunneling through IPv4, referred to as 6to4 (see Figure 8.17). 6to4 specifies a new technique designed for interconnecting IPv6 networks over IPv4 networks without explicitly defined tunnels. The IPv4 address (V4ADDR) portion of the special 6to4 Applications Applications Sockets API UDP/TCP v4 UDP/TCP v6 Figure 8.16 Dual Stack Chapter 8 The Internet and IP Infrastructures IPv6 SPrefix-2002:V4ADDR::48 2002::16 IPv4 Address-V4ADDR Private 6to4 IPv6 Site Network Network Border Router Relay Router (Dual Stac