During the IPv6 auto configuration, what does the device append to the 64-bit prefix that it receives from the router to create its IPv6 address?

A. a pseudorandom generated number

B. its locally configured IPv4 address

C. the DHCP-supplied device ID

D. its MAC address D








 

Suggested Answer: Explanation

The automatic configuration is a great feature of IPv6. Imagine you have to manually configure an IPv6 address with 128-bit long, what a pain!
With this feature, it is no longer necessary to configure each host manually. But notice that host only autonomously configures its own Link- local address (the IP address used on a LAN). The Link-local address can be created automatically using a link-local prefix of FE80::/10 and a 64-bit interface identifier (based on 48-bit
MAC address).
For example, if your MAC address is 00:12:34:56:78:9a, your 64-bit interface identifier is 0012:34FF:FE56:789a (16-bit FFFE is inserted in the middle). And notice that the notation has been changed because IPv6 addresses require 16-bit pieces to be separated by ":".
Then, according to the RFC 3513 we need to invert the Universal/Local bit ("U/L" bit) in the 7th position of the first octet (start counting from 0). The "u" bit is set to
1 to indicate Universal, and it is set to zero (0) to indicate local scope. In this case we set this bit to 1 because the MAC address is universally unique. Thus the result is: 0212:34FF:FE56:789a.
Finally, add the link-local prefix FE80 to create the full IPv6 address: FE80:0:0:0:0212:34FF:FE56:789a (or FE80::212:34FF:FE56:789a in short form)
Note: The reason for inverting the "U/L" bit is to allow ignoring it for short values in the manual configuration case. For example, you can manually assign the short address fc80::1 instead of the long fc80:0:0:0:0200::1

This question is in 300-101 Cisco Implementing Cisco IP Routing (ROUTE) Exam
For getting Cisco Certified Network Professional (CCNP) Routing and Switching Certificate




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