IPv6 Link Local address and Unique Local Address

IPv6 Unique Local Address ULA
  • link local addresses are only significant on a single link
  • link local addresses can overlap on the same router as long as defined on different interfaces
  • starts with FE80::/10
  • link local addresses are mainly used by Ipv6 routing protocol, on broadcast networks link local addresses are automatically revolved through ICMP ND (ICMP Neighbor Discovery) since on non-broadcast this can’t be done this has to be manually mapping
  • Link local addresses if not configured manually will be driven from the bia of the interface EUI Extended Unique Identifier 
For example if the bia of an interface is as follows
R2(config-if)#do sh interf f0/0 | in add
  Hardware is Gt96k FE, address is c201.42b8.0000 (bia c201.42b8.0000)
1- FFEE is inserted after the vendor ID (most significant 3 bytes)
C201.42FF.FEb8.0000
b- invert the 7th most significant bit, so 
0x C2 is 1100 0010 binary
invert the 1 to 0 , so this becomes 1100 0000 which is 0x C0
so our address is FE80:C001:42FF:FEB8:0
which can be verified by checking the ipv6 local link address for that interface
R2(config-if)#do sh ipv6 interface f0/0 | in lin
FastEthernet0/0 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::C001:42FF:FEB8:0
  No Virtual link-local address(es):

IPv6 Unique Local Address ULA
  • Unique Local Ipv6 address replaced previously used site local IPv6 address
  • ULA IPv6 addresses are equivalent to private IPv4 address, these addresses are not routable on the global routing table
  • ULA has the following format
FC00 (7bits) + Unique ID (41 bits) + link ID (16 bits) + Interface ID (64 bits)

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