I came across this, great site for online network troubleshooting..

 

W3dt.Net(https://w3dt.net/)

I regularly use this for defining CIDR for within VPC.

https://w3dt.net/tools/cidrnetmask

Some Notes on AWS VPC:

To add a CIDR block to your VPC, the following rules apply:

  • The allowed block size is between a /28 netmask and /16 netmask.
  • The CIDR block must not overlap with any existing CIDR block that’s associated with the VPC.
  • There are restrictions on the ranges of IPv4 addresses you can use. For more information, see IPv4 CIDR Block Association Restrictions.
  • You cannot increase or decrease the size of an existing CIDR block.

http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html

 

Examples:

w3dt.net CIDR Netmask Calculator. (10.0.0.0/8)
-------------------------------------------------------
Address: 10.0.0.0
Netmask: 255.0.0.0 = 8
Network: 10.0.0.0
Broadcast: 10.255.255.255
HostMin: 10.0.0.1
HostMax: 10.255.255.254
Hosts/Net: 16777214
w3dt.net CIDR Netmask Calculator.(10.0.0.0/16)
-------------------------------------------------------
Address: 10.0.0.0
Netmask: 255.255.0.0 = 16
Network: 10.0.0.0
Broadcast: 10.0.255.255
HostMin: 10.0.0.1
HostMax: 10.0.255.254
Hosts/Net: 65534
w3dt.net CIDR Netmask Calculator.(10.0.0.0/32)
-------------------------------------------------------
Address: 10.0.0.0
Netmask: 255.255.255.255 = 32
Network: 10.0.0.0
Broadcast: N/A
HostMin: N/A
HostMax: N/A
Hosts/Net: N/A
w3dt.net CIDR Netmask Calculator. (10.0.0.0/24)
-------------------------------------------------------
Address: 10.0.0.0
Netmask: 255.255.255.0 = 24
Network: 10.0.0.0
Broadcast: 10.0.0.255
HostMin: 10.0.0.1
HostMax: 10.0.0.254
Hosts/Net: 254
w3dt.net CIDR Netmask Calculator. (10.0.0.0/28)
-------------------------------------------------------
Address: 10.0.0.0
Netmask: 255.255.255.240 = 28
Network: 10.0.0.0
Broadcast: 10.0.0.15
HostMin: 10.0.0.1
HostMax: 10.0.0.14
Hosts/Net: 14