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.

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11 comments

  1. muslimtech

    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

  2. muslimtech

    Hi Pashtuk

    7 bit in mac address = bit number 71 in ipv6 address “if” i used Eui-64 method

    I mean , what is different?

  3. Pingback: IPv6 Address Layers - TechExams.net IT Certification Forums
    • Vikas

      Hi,

      How you will convert 0008.209a.081b in link-local address. First step is ok where u insert FFFE i.e. 0008.20FF.FE9a.081b
      issue is 2nd step, how to convert 0008 to binary?
      I have done recerse engineering from link-local address
      link-local address is FE80::208:20FF:FE9A:81B

      when u convert 208 to binary it comes 10000001000. So my question is how to convert 0008 to 10000000000 (this comesto 400 in HEX)
      Regards,
      Vikas

  4. Gaurav Bansal

    Vikas
    you forgot to add the leading zeros in 0x208
    it should be 0x0208 which is 0000 0010 0000 1000 which is the same as 10000001000 minus the leading zeros
    hope that solves ur query

    Gaurav

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