Information Technology Reference
In-Depth Information
RFC 3587. RFC 3587 simplifies these identifiers with a global routing prefix and subnet
identifier for the network portion of the address.
Figure 9-2 shows the format of the standard IPv6 global unicast address. The global rout-
ing prefix is generally 48 bits in length, and the subnet ID is 16 bits. The interface ID is 64
bits in length and uniquely identifies the interface on the link.
Network Portion
Host Portion
48 Bits
16 Bits
64 Bits
Global Routing Prefix
Subnet ID
Interface ID
Figure 9-2
IPv6 Global Unicast Address Format
The interface ID is obtained from the 48-bit MAC address of the host. The MAC is con-
verted to the EUI-64 identifier format by inserting the FFFE hexadecimal value in be-
tween the 24-bit left and rightmost values.
For example, w ith the follow ing MAC addres s 01:00:0C:A4:BC:D0. The leftmost 24 bits
are 01:00:0C and the rightmost bits are 01:00:0C. By inserting FFFE the IPv6 64bit identi-
fier becomes:
01:00:0C:FF:FE:A4:BC:D0.
Link-Local Addresses
IPv6 link-local addresses are significant only to nodes on a single link. Routers do not for-
ward packets with a link-local source or destination address beyond the local link. Link-lo-
cal addresses are identified by leading FE8 hexadecimal numbers. Link-local addresses are
configured automatically or manually.
As shown in Figure 9-3, the format of the link-local address is an FP of 1111111010, fol-
lowed by 54 0s and a 64-bit interface identifier (ID). The interface ID is obtained from the
device MAC address and verified automatically through communication with other nodes
in the link. The interface ID is then concatenated with the link-local address prefix of
FE80::/64 to obtain the interface link-local address.
10 Bits
54 Bits
64 Bits
1111 1110 10
0
Interface ID
Figure 9-3
IPv6 Link-Local Address Format
Unique Local IPv6 Address
 
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