IP address and Subnetting

feature article



Private IP addressing


binary bits

Subnet Mask calculation

If the CIDR value is /25 then subnet mask will be – In binary – 11111111.11111111.11111111.10000000  (8+8+8+1 = 25)

Same way if subnet mask is – the CIDR value will be /18 (8+8+2)

Wild card calculation –

Whatever the subnet mask minus it with 255. E.g. – mask wildcard will be

wild card calc

Supernet calculation

Sometimes we set lesser subnet mask or cidr value than the default value & apply it on ACL, Prefix-list or route advertisement. This will help us to just select 1 master subnet & all child subnet will be select. Example –

  1. – This will select to         B. 13 will select to

Master network octet + wildcard mask octet = extended range

ext range

If we extend master subnet to then calculation of subnet mask or cidr value will be 31-16 = 15. So calculating by wildcard – mask will be – Extended range minus master range = wildcard octet (convert into subnet mask)

wildcard to select

Note that we cannot select any range from our mind, e.g. if we want to range from to then we’ve to start from instead of