Users Guide
• 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 dened in the Next Header eld of the preceding IPv6
header.
IPv6 Header Fields
The 40 bytes of the IPv6 header are ordered, as shown in the following illustration.
Figure 44. IPv6 Header Fields
Version (4 bits)
The Version eld always contains the number 6, referring to the packet’s IP version.
Trac Class (8 bits)
The Trac Class eld deals with any data that needs special handling. These bits dene the packet priority and are dened by the packet
Source. Sending and forwarding routers use this eld to identify dierent IPv6 classes and priorities. Routers understand the priority
settings and handle them appropriately during conditions of congestion.
Flow Label (20 bits)
The Flow Label eld identies packets requiring special treatment in order to manage real-time data trac.
The sending router can label sequences of IPv6 packets so that forwarding routers can process packets within the same ow without
needing to reprocess each packet’s header separately.
NOTE
: All packets in the ow must have the same source and destination addresses.
IPv6 Routing 351