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TCP/IP SUITE AND INTERNET STACK PROTOCOLS 37
Table 2.5 Type prefixes for IPv6 addresses
Type prefix
(binary)
Type of address Fraction of
address space
0000 0000 Reserved 1/256
0000 0001 Reserved 1/256
0000 001 NSAP (Network Service Access
Point)
1/128
0000 010 IPX (Novell) 1/128
0000 011 Reserved 1/128
0000 100 Reserved 1/128
0000 101 Reserved 1/128
0000 110 Reserved 1/128
0000 111 Reserved 1/128
0001 Reserved 1/16
001 Reserved 1/8
010 Provider-based unicast addresses 1/8
011 Reserved 1/8
100 Geographic unicast addresses 1/8
101 Reserved 1/8
110 Reserved 1/8
1110 Reserved 1/16
1111 0 Reserved 1/32
1111 10 Reserved 1/64
1111 110 Reserved 1/128
1111 1110 0 Reserved 1/512
1111 1110 10 Link local addresses 1/1024
1111 1110 11 Site local addresses 1/1024
1111 1111 Multicast addresses 1/256
Prefix (variable)
Rest of address (variable)
128 bits
occupy up to 65 535 bytes of information. Figure 2.8 shows the base header with its eight
fields. Each IPv6 datagram begins with a base header. The IPv6 header has a fixed length
of 40 octets, consisting of the following fields:
• Version: This four-bit field defines the version number of the IP. For IPv6, the
value is 6.
• Priority: This four-bit priority field defines the priority of the packet with respect to
traffic congestion. So, this field is a measure of the importance of a datagram. The
IPv4 service class field has been renamed the IPv6 traffic class field.
• Flow label: This 24-bit field is designed to provide special handling for a particular
flow of data. This field contains information that routers use to associate a datagram
with a specific flow and priority.