172.128.0.0 it's a class B address and you are subnetting it from /16 to /26 which means you are using 10 bits from the host address towards subnet, so your total no of subnets = 2 raise to power N ( N is 10 in your care ) so its 1024.
Also the above mentioned subnets that you created in Red are in correct for /26 mask , they should be :
172.128.0.0/26
172.128.0.64/26
172.128.0.128/26
172.128.0.192/26
Each of the above gives you 64 Host and if you continue on the same logic you will get to 1024 subnets. :-)
Manish
More Info :
http://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.html
