/25:
CIDR to IP Range
Result
CIDR Range 172.16.20.128/25
Netmask 255.255.255.128
Wildcard Bits 0.0.0.127
First IP 172.16.20.128
First IP (Decimal) 2886734976
Last IP 172.16.20.255
Last IP (Decimal) 2886735103
Total Host 128
CIDR
172.16.20.128/25
/24:
CIDR to IP Range
Result
CIDR Range 172.16.20.128/24
Netmask 255.255.255.0
Wildcard Bits 0.0.0.255
First IP 172.16.20.0
First IP (Decimal) 2886734848
Last IP 172.16.20.255
Last IP (Decimal) 2886735103
Total Host 256
CIDR
172.16.20.128/24

Managed instance groups offer autoscaling capabilities that let you automatically add or delete instances from a managed instance group based on increases or decreases in load (CPU Utilization in this case). Autoscaling helps your apps gracefully handle increases in traffic and reduce costs when the need for resources is lower.
You define the autoscaling policy and the autoscaler performs automatic scaling based on the measured load (CPU Utilization in this case). Autoscaling works by adding more instances to your instance group when there is more load (upscaling), and deleting instances when the need for instances is lowered (downscaling). Ref:
https://cloud.google.com/compute/docs/autoscaler

https://cloud.google.com/compute/docs/instances/managing-instance-access