Regarding the prefix that are covered in 128.0.0.0/3 & 128.0.0.0/5.
Why does it stop at subnet #6 for /3 and #19 for /5
256 – 224 = 32 subnets
256 – 248 = 8 subnets
So for /3 it looks like this:
Subnet #1: 0.0.0.0/3
Subnet #2: 32.0.0.0/3
Subnet #3: 64.0.0.0/3
Subnet #4: 96.0.0.0/3
Subnet #5: 128.0.0.0/3
Subnet #6: 160.0.0.0/3
And for /5:
Subnet #1: 0.0.0.0/5
Subnet #2: 8.0.0.0/5
Subnet #3: 16.0.0.0/5
Subnet #4: 24.0.0.0/5
Subnet #5: 32.0.0.0/5
Subnet #6: 40.0.0.0/5
Subnet #7: 48.0.0.0/5
Subnet #8: 56.0.0.0/5
Subnet #9: 64.0.0.0/5
Subnet #10: 72.0.0.0/5
Subnet #11: 80.0.0.0/5
Subnet #12: 88.0.0.0/5
Subnet #13: 96.0.0.0/5
Subnet #14: 104.0.0.0/5
Subnet #15: 112.0.0.0/5
Subnet #16: 120.0.0.0/5
Subnet #17: 128.0.0.0/5
Subnet #18: 136.0.0.0/5
Subnet #19: 144.0.0.0/5