IPv6 Link-Local Address
Link-local addresses only have a scope of the link they are used on, they will not be used or even reachable from another link.
The link-local address of an interface is a mix between the BIA (Burned In Address) and some static IPv6 stuff. The “dynamic” part is also know as modified EUI-64 addressing.
First of all, every link-local address in IPv6 is generated from FE80::/10, so if you ever see a address starting with FE80 you know thats a link-local address 
Over a sh int you see your BIA:
R1#sh int fa 0/0
FastEthernet0/0 is up, line protocol is up
Hardware is AmdFE, address is cc00.0bfc.0000 (bia cc00.0bfc.0000)
The BIA in this case is cc00.0bfc.0000. The rules for the modified EUI-64 addressing are:
- FFFE will be put in between the vendor-id (3 most significant bytes) and the extension-id (3 least significant bytes), which will lead to cc00.0bFF.FEfc.0000
- Thereafter the seventh bit, known as universal/local bit, gets inverted. So first change the cc00 from hex to binary, which leads to 110011 0 000000000 and then invert the seventh bit (here a 0): 110011 1 000000000. Moved back to hex this will give CE00
- At last, change all points ‘.’ to colons ‘:’ and delete all leading zeros, then you got your link-local IPv6 address: FE80::CE00:BFF:FEFC:0
R1#sh ipv6 int fa 0/0
FastEthernet0/0 is up, line protocol is up
IPv6 is enabled, link-local address is FE80::CE00:BFF:FEFC:0
No global unicast address is configured
Joined group address(es):
FF02::1
FF02::2
FF02::1:FFFC:0
MTU is 1500 bytes
ICMP error messages limited to one every 100 milliseconds
ICMP redirects are enabled
ND DAD is enabled, number of DAD attempts: 1
ND reachable time is 30000 milliseconds
Its also possible to create a global unique IPv6 address with the modified EUI-64 addressing, you just have to add the keyword eui-64 after the ipv6 address command:
ipv6 address 2001::/64 eui-64
If you do not want to use the eui-64 addressing scheme for your link-local addresses, you could also change it it:
ipv6 address FE80::1 link-local
But remember link-local has always to start with FE80.
7 Comments »
Leave a comment
| Next »
About
Im a network engineer working for an international medical company based in Basel, Switzerland. I began my professional life in 2000 after 4 years of an IT apprenticeship. I started as the Network Management System responsible with quite a lot other of other tasks inside the Cisco networking field. In 2005 I changed to my current employer where I changed my field of knowledge from part NMS part Networking to full time Networking. I also started to educate myself deeper into networking. After passing the CCNA and CCNP exams I wanted to go further with my education and took the CCIE R&S track instead of doing a school with lots of stuff Im not interested in 
I started the CCIE journey in August 2007 with the first of 5 workshops at the Cisco Network Academy in Rapperswil and passed the CCIE in Brussels at 07th of May 2008.
During that time I wrote a lot of documents to keep the stuff I learned and I thought it might be an idea to make it public so it might help other people on their path through the Cisco Certifications. I hope this site gets useful not only for CCIE candidates but also for other networking professionals on their search for knowledge.
If you have any questions or want to contact me, please drop me a line to pashtuk at gmail.com
hi man
i want ask you Q About U\L Bit
i read that in Bsci
Universal/Local (U/L)
The seventh bit in an IPv6 interface identifier is referred to as the universal/local bit, or U/L bit. This bit identifies whether this interface identifier is universally or locally administered.
If the U/L bit is set to 0, the address is locally administered. The network administrator has overridden the manufactured address and specified a different address.
If the U/L bit is set to 1, the IEEE, through the designation of an ISP, has administered the address
i cant understand How U\L Bit = 0 i want Ex; About that
and whene he say “locally administered” mean
i use command to change mac address for interface ??
or what
Hi
I’ll have to digg into this but as far as I can see you’re probably talking about the 71st and not the 7th bit?
Hi Pashtuk
7 bit in mac address = bit number 71 in ipv6 address “if” i used Eui-64 method
I mean , what is different?
Yes, you’re right, 71st bit is equal to the 7th bit of a MAC address.
I’m still researching that stuff but its all a bit spongy…
thank you man this bit will make me mad
Hi pashtuk
Did You find answer ?
Sorry, did not have any time to go into that at the moment. To much stuff to do at work, hope to get into it soon.