Subnet Router Anycast

[ChrisNewnham_]

I understand why ::0, 4 and 8 and multiples of 4 are subnet router anycast addresses, but not 10, 14, and 18. Can anyone explain this?

Thanks

r4(config-if)# ipv6 address 2001:100::/126
%GigabitEthernet0/0: Warning: 2001:100::/126 is a Subnet Router Anycast
r4(config-if)# ipv6 address 2001:100::1/126
r4(config-if)# ipv6 address 2001:100::2/126
r4(config-if)# ipv6 address 2001:100::3/126
r4(config-if)# ipv6 address 2001:100::4/126
%GigabitEthernet0/0: Warning: 2001:100::4/126 is a Subnet Router Anycast
r4(config-if)# ipv6 address 2001:100::5/126
r4(config-if)# ipv6 address 2001:100::6/126
r4(config-if)# ipv6 address 2001:100::7/126
r4(config-if)# ipv6 address 2001:100::8/126
%GigabitEthernet0/0: Warning: 2001:100::8/126 is a Subnet Router Anycast
r4(config-if)# ipv6 address 2001:100::9/126
r4(config-if)# ipv6 address 2001:100::10/126
%GigabitEthernet0/0: Warning: 2001:100::10/126 is a Subnet Router Anycast
r4(config-if)# ipv6 address 2001:100::11/126
r4(config-if)# ipv6 address 2001:100::12/126
r4(config-if)# ipv6 address 2001:100::13/126
r4(config-if)# ipv6 address 2001:100::14/126
%GigabitEthernet0/0: Warning: 2001:100::14/126 is a Subnet Router Anycast
r4(config-if)# ipv6 address 2001:100::15/126
r4(config-if)# ipv6 address 2001:100::16/126
r4(config-if)# ipv6 address 2001:100::17/126
r4(config-if)# ipv6 address 2001:100::18/126
%GigabitEthernet0/0: Warning: 2001:100::18/126 is a Subnet Router Anycast
r4(config-if)# ipv6 address 2001:100::19/126
r4(config-if)# ipv6 address 2001:100::20/126
%GigabitEthernet0/0: Warning: 2001:100::20/126 is a Subnet Router Anycast

[Harold Ritter]

Hi @ChrisNewnham_ ,

This is because the ipv6 addresses are expressed in hexadecimal, rather than decimal for ipv4 addresses.

So ::10/126 is actually binary 0001 0000 and it is considered a anycast address since the last two bits are zero. The same goes for ::14/126 and ::18/126.

::14/126 = bin 0001 0100

::18/126 = bin 0001 1000

Regards,

[ChrisNewnham_]

Thanks, realised now I should have gone 2,6,A,D etc instead of 2,6,10,14

[Harold Ritter]

Hi @ChrisNewnham_ ,

Since the /126 entails groups of 4 addresses, the subnet router anycast address sequence would be like this.

0,4,8,c,10,14,18,1c,20,24,28,2c...

Regards,

Regards,

[ChrisNewnham_]

Sorry I was talking about using the second available host address in each /126 subnet - thanks anyway though!