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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. 11111000. 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. 11111000. 00000000 = /21
11111111. 11111111. 11110000. 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