IPv6 Routing | 539
IPv6 Headers
The IPv6 header has a fixed length of 40 bytes. This provides 16 bytes each for Source and Destination
information and 8 bytes for general header information. The IPv6 header includes the following fields:
• Version (4 bits)
• 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 25-1.
Figure 25-1. IPv6 Header Fields
Version (4 bits)
The Version field always contains the number 6, referring to the packet’s IP version.
048121620242832
64
320
192
128
Version Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source Address
Destination Address
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