Network Utilities

IP Lookup, CIDR Calculator, MAC Address Lookup, and Port Number Lookup
Examples:

Subnet Calculator

Here's the subnet information about the given CIDR.
Subnet Range 216.73.216.0 through 216.73.216.255
Subnet Size 256 IPs
Netmask 255.255.255.0
Bitmask 24
API Call

Get JSON results via command line with either curl or wget.

Try it:
curl "https://steamr.com/?q=216.73.216.137/24" 

{
    "start": "216.73.216.0",
    "end": "216.73.216.255",
    "bits": 24,
    "netmask": "255.255.255.0",
    "size": 256
}