IPv6 Addressing | 487Hop Limit (8 bits)The Hop Limit field shows the number of hops remaining for packet processing. In IPv4, this is known asthe Time to Live (TTL) field and uses seconds rather than hops.Each time the packet moves through a forwarding router, this field decrements by 1. If a router receives apacket with a Hop Limit of 1, it decrements it to 0 (zero). The router discards the packet and sends anICMPv6 message back to the sending router indicating that the Hop Limit was exceeded in transit.Source Address (128 bits)The Source Address field contains the IP address for the packet originator.Destination Address (128 bits)The Destination Address field contains the intended recipient’s IP address. This can be either the ultimatedestination or the address of the next hop router.Extension Header fieldsExtension headers are used only when necessary. Due to the streamlined nature of the IPv6 header, addingextension headers do not severely impact performance. Each Extension headers’s lengths vary, but they arealways a multiple of 8 bytes.Each extension header is identified by the Next Header field in the IPv6 header that precedes it. Extensionheaders are viewed only by the destination router identified in the Destination Address field. If theDestination Address is a multicast address, the Extension headers are examined by all the routers in thatmulticast group.However, if the Destination Address is a Hop-by-Hop options header, the Extension header is examinedbye every forwarding router along the packet’s route. The Hop-by-Hop options header must immediatelyfollow the IPv6 header, and is noted by the value 0 (zero) in the Next Header field (Table 22-1).Extension headers are processed in the order in which they appear in the packet header.59 No Next Header60 Destinations option headerNote: This is not a comprehensive table of Next Header field values. Refer to the InternetAssigned Numbers Authority (IANA) web page http://www.iana.org/assignments/protocol-numbersfor a complete and current listing.Table 22-1. Next Header field values (continued)Value Description
