**In Binary Net Masking each Octet contains exponential bits from right to left:**

1 1 1 1 1 1 1 1

128 64 32 16 8 4 2 1 (128+64+32+16+8+4+2+1=**255**)

**So..**

255.255.248.0 in binary is

11111111. 11111111. 11111**000**. **00000000** = /21 (we’ll get to the identifier in a minute)

**255-248 = 7**

Therefore 255.255.248.0 – supports 7 networks per subnet

255 – 248 = 7 networks

**Or**

**192.168.1.0/21** supports

192.168.1.0-255

*through*

192.168.7.0-255

** The next subnet supports another 7 networks.**

192.168.8.0/21

192.168.8.0-255

* through*

192.168.15.0-255

**So where does the /21 identifier come from?**

All net mask identifiers start at /32 and go down.

32-21 = 11 mask bits

**Count the zeroes in the Binary Mask!**

11111111. 11111111. 11111**000. 00000000** = **/21**

11111111. 11111111. 1111**0000. 00000000** = **/20** (32-20 = 12 mask bits)

**What’s the full net mask for the /20 identifier above?**

1 1 1 1 1 1 1 1

128 64 32 16 8 4 2 1 (8+4+2+1=**15**)

**/20 = 255.255.240.0** *because ***255**–**15** = **240**