Subnet Increase - Scope Expansion

[Dan Man]

I'm either not thinking about this properly, or forgot how to subnet.

I have a network of 10.40.68.0 255.255.252.0. I'm running out of address space on it so I want to increase it. I want the mask to be 255.255.248.0. This would make the new scope 10.40.68.1-10.40.75.254. However when I try to add this I'm being told that the mask and network aren't correct. That the scope will be 10.40.64.1-10.40.71.254. Am I missing something here? Can someone explain why this is? Your help is greatly appreciated.

[GRANT3779 · Accepted solution]

your subnet mask is 21 bits

3rd Octet is 248

128 64 32 16 8 = 248

(going up multiples of 8)

10.40.0.1 - 10.40.7.254 - Network 1
10.40.8.1 - 10.40.15.254 - Network 2
10.40.16.1 - 10.40.23.254 - Network 3
10.40.24.1 - 10.40.31.254 - Network 4
10.40.32.1 - 10.40.39.254 - Network 5
10.40.40.1 - 10.40.47.254 - Network 6
10.40.48.1 - 10.40.55.254 - Network 7
10.40.56.1 - 10.40.63.254 - Network 8

Next network starts 10.40.64.0 

So your 10.40.68.0 subnet falls within the 10.40.64.0 /20 range

(usable addresses - 10.40.64.1 - 10.40.71.254)