Concept Guide

• 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 dened 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 59. IPv6 Header Fields
Version (4 bits)
The Version eld always contains the number 6, referring to the packet’s IP version.
Trac Class (8 bits)
The Trac Class eld deals with any data that needs special handling. These bits dene the packet priority and are dened by the packet
Source. Sending and forwarding routers use this eld to identify dierent 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 identies packets requiring special treatment in order to manage real-time data trac.
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.
Payload Length (16 bits)
The Payload Length eld species the packet payload. This is the length of the data following the IPv6 header. IPv6 Payload Length only
includes the data following the header, not the header itself.
The Payload Length limit of 2 bytes requires that the maximum packet payload be 64 KB. However, the Jumbogram option type Extension
header supports larger packet sizes when required.
IPv6 Routing
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