Reference Guide

IPv6 Routing | 547
• Traffic Class (8 bits)
• Flow Label (20 bits)
• Payload Length (16 bits)
• Next Header (8 bits)
• Hop Limit (8 bits)
• Source Address (128 bits)
• Destination Address (128 bits)
IPv6 provides for Extension Headers. Extension Headers are used only if necessary. There can be no
extension headers, one extension header or more than one extension header in an IPv6 packet. Extension
Headers are defined in the Next Header field of the preceding IPv6 header.
IPv6 Header Fields
The 40 bytes of the IPv6 header are ordered as show in Figure 26-1.
Figure 26-1. IPv6 Header Fields
Version (4 bits)
The Version field always contains the number 6, referring to the packet’s IP version.
Traffic Class (8 bits)
The Traffic Class field deals with any data that needs special handling. These bits define the packet priority
and are defined by the packet Source. Sending and forwarding routers use this field to identify different
IPv6 classes and priorities. Routers understand the priority settings and handle them appropriately during
conditions of congestion.
048121620242832
64
320
192
128
Version Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source Address
Destination Address