Professor Messer 1.4: Magic Number Subnetting

Network address / Subnet ID

The first address in the subnet

Broadcast address

The last address in the subnet

First available host address

One more than the network address

Last available host address

One less than the broadcast address

CIDR for interesting octet 2

/9 through /16

CIDR for interesting octet 3

/17 through /24

CIDR for interesting octet 4

/25 through /30

Magic Number

128 64 32 16 8 4 2 1

Find the subnet ID

Copying the IP address wherever the mask is 255. If the mask is 0, copy 0 instead. When the mask is neither 255 nor 0, identify this as the interesting octet. Subtract the interesting octet mask from 256 to get the magic number. This number determines the subnet ranges (e.g., a result of 16 gives ranges of 0, 16, 32, 48, etc.). Look at the IP address, and round down to the nearest multiple of the magic number.

Find the broadcast address

Copying the subnet ID wherever the mask is 255. If the mask is 0, write 255. When the mask is neither 255 nor 0, identify this as the interesting octet. Calculate the magic number by subtracting the interesting octet mask from 256. Add the magic number to the subnet ID and then subtract 1.

Find the host range

The first host is the subnet ID plus 1, and the last host is the broadcast address minus 1.

Subnet mask for interesting octet

128

192

224

240

248

252

254

255