Sunday, 28 October 2012

IPv6: Understanding Modified EUI-64 addressing

IPv6: Understanding Modified EUI-64 addressing


·         In IPv6, Link-local address is automatically assigned.

·         Link-local address always starts with FE80

·         For the Network portion, it is 1111 1110 10 followed by 54 bits of 0, total 64 bits.

·         Host portion is determined by MAC

·         But MAC address is only 48 bits??

·         It inserts FFFE in between and toggles in 7th MSB bit

·         Ex: MAC is 1234.5678.9123

·         Modified EUI: 1234.56FF.FE78.9123

·         Let me write first 8 bits of 1234.56FF.FE78.9123 in binary, we get 00010010

·         If I toggle the 7th MSB, then it is 0001 0000

·         So, the host portion would be 1034.56FF.FE78.9123

·         This is called, modified EUI-64 addressing


·         Let me take a router and see how this happens:


R1(config)#int fa0/0

R1(config-if)#ipv6 enable


R1(config-if)#do show ipv6 int fa0/0

FastEthernet0/0 is up, line protocol is up

  IPv6 is enabled, link-local address is FE80::C000:1FFF:FEB4:0

  No global unicast address is configured

  Joined group address(es):




  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






·         As soon as I enabled ipv6 under fa0/0, it automatically assigned link-local address

·         And the link-local address is FE80::C000:1FFF:FEB4:0

·         Network portion is FE80 and the host portion is C000:1FFF:FEB4:0


·         To see how its determined host portion, let me check the MAC address of the fa0/0


R1(config-if)#do show int fa0/0 |  i bia

  Hardware is Gt96k FE, address is c200.1fb4.0000 (bia c200.1fb4.0000)



·         If we see the above output, Fa0/0 MAC is c200.1fb4.0000

·         It’s inserted FFFE in the middle and toggled the 7th MSB

·         First 8 bits of C200.1fff.feb4.0000 : 1100 0010 and if I toggle the 7th MSB then it is 1100 0000, i.e., C000:1FFF:FEB4:0

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