The Essentials

This page hosts a CIDR Network Calculator that you can download and run on any computer with Python version 3.x. CIDR stands for "Classless Inter-Domain Routing", and represents the preferred method of carving up IPV4 networks for the last 15 years. You can view or download the calculator here.

To use this calculator to find a small IP address group containing addresses between 192.168.47.21 and 192.168.47.67, issue the following command:

[slitt@mydesk ~]$ ./cidr_calc.py  192.168.47.21/24

Original ip spec      : 192.168.47.21/24
CIDR Network Notation : 192.168.47.0/24
Netmask               : 255.255.255.0
IP range              : 192.168.47.0 - 192.168.47.255
Chunk size            : 256 (2^8) consecutive IPs

[slitt@mydesk ~]$

The /24 at the end was too inclusive, so trial and error /26 instead:

[slitt@mydesk ~]$ ./cidr_calc.py 192.168.47.21/26

Original ip spec      : 192.168.47.21/26
CIDR Network Notation : 192.168.47.0/26
Netmask               : 255.255.255.192
IP range              : 192.168.47.0 - 192.168.47.63
Chunk size            : 64 (2^6) consecutive IPs

[slitt@mydesk ~]$

Whoops, that's not inclusive enough because it stops short of 67. So split the difference:

[slitt@mydesk ~]$ ./cidr_calc.py 192.168.47.21/25

Original ip spec      : 192.168.47.21/25
CIDR Network Notation : 192.168.47.0/25
Netmask               : 255.255.255.128
IP range              : 192.168.47.0 - 192.168.47.127
Chunk size            : 128 (2^7) consecutive IPs

[slitt@mydesk ~]$

This calculator was deliberately made to be easily used with trial and error, so you can get the range you want to get.

Once again, you can view or download the calculator at http://troubleshooters.com/linux/cidr_calc.py.txt.

Glossary of CIDR Terms

CIDR: "Classless Inter-Domain Routing", the standard method of carving up network address spaces.
Classless Inter-Domain Routing: Carving up the address spaces without classes. Before the commoditization of the Internet in the mid 1990's, companies and organizations were awarded chunks of IP address space in one of these three quantities:
- 2⁸ (256) consecutive addresses, a class C network.
- 2¹⁶ (65,536) consecutive addresses, a class B network.
- 2²⁴ (16,777,216) consecutive addresses, a class A network.
With such huge jumps, it's no wonder people ordered way to many addresses "just in case". Too quickly we began to approach the 2³² (4,294,967,296) maximum number of assignable addresses, so everyone changed to classless, so IP ranges could be assigned in any power of 2.
IPV4: An Internet addressing protocol using 2³² (2 to the 32nd power) different addresses. These addresses are represented as four numbers, each between 0 and 255, separated by dots. IPV4 is slowly being replaced by IPV6, but there's reason to believe that at the Local Area Network (LAN) level IPV4 will continue being used for a long time.
IPV6: An Internet addressing protocol using 2¹²⁸, less some stretches used for special things. IPV6 is not relevant to this document, nor is this document relevant to IPV6.
Internet Address: A representation of a number usually unique to a specific computer, used to identify the source and destination computers of a given piece of communication. The reason it's "usually unique" is because many organizations use private network IP addresses, and different organizations can use the same private network IP addresses.
IP Address: Synonym for Internet Address
Private network IP address: In IPV4 addressing, a private IP address is one that falls into one of these three ranges:
- 192.168.0.0/16
- 172.16.0.0/12
- 10.0.0.0/8
To fully understand the preceding ranges, run them through the CIDR Network Calculator to understand ranges, bitmasks, and the like. Note that the private addresses starting with 192 are often used to form subnets of 256 different addresses instead of 65566 addresses. The 192.168.47.0/24 network is an example.
Subnet: A group of IP addresses whose computers can talk wto each others without routers. If you type an IPaddress/maskbits specification into the CIDR calculator, the IP range the calculator gives you back is a subnet. The less bits in the maskbits part of the specification, the less addresses are in the subnet.
Maskbits: The number after the slash in a network specification. So in 192.168.47.0/26, the maskbits value is 26. Maskbits can go between 0 and 32. In practice the value is never below 8, and never above 31, because 32 means a subnet of one computer, and if there's one computer, what's the use of networking?
Bitmask: An older way of showing the size of a network. Like the Maskbits specification, Bitmask depends solely on the size of the network chunk. Maskbits and Bitmask can be calculated from each other.

Best Practices for Using This CIDR Calculator

This calculator was built from the ground up for trial and error and "what if" usage. It was made assuming you have a "history" command so you can recall the preceding command with the uparrow key and modify the preceding command as needed. So start with the bottom-most IP address you want to cover, and keep adjusting the maskbits figure (the number after the slash), until you cover all the IP addresses you want..

This calculator isn't a spreadsheet. You can't modify any given variable and see all the rest change. The only variables you can modify are the IP address and the mask bits. The calculator then prints out all the rest of the information. The ip_address/maskbits format for the argument is used because it's simple and memorable.

If you have Python 3.x installed on your computer, whether Linux, BSD, Mac or Windows, you should be able to operate this calculator with the following command (in a terminal or command processor):

python3 ./cidr_calc.py  10.10.0.0/20

Obviously, substitute what you need for 10.10.0.0/20. If the command python3 doesn't run Python, version 3, for you, then find the command that does. You must have Python 3.x for this program to work: Python 2.x is deprecated and will cease all support 1/1/2020: 15 months from when this web page was first authored.

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