subnet-py

A simple subnetting tool written in Python.
This tool is designed to take an IP address and a CIDR prefix and calculate the Netword Address, Start/End Address Range, Usable Start/End Addresses, Total Usable Hosts and the Broadcast Address. How it does it, it's explained in the next Chapter.

Running examples

$ python3 subnet.py 20.10.20.10 13
Subnet Address: 20.10.20.10/13
Range of IP Addresses: 20.8.0.0 - 20.15.255.255
Usable IPs: 20.8.0.1 - 20.15.255.254
Hosts: 524286
Broadcast IP: 20.15.255.255

How it does the calculations

• Start(First) Address of the Subnet:
  The start address is calculated by doing a bitwise AND operation between the given IP address and the CIDR Subnet Mask.
  ip & masks[cidr] in line 47 in the code.

• End(Last/Broadcast) Address of the Subnet:
  This is a tricky calculation, but generaly, this address is calcuated by bitwise OR between the given IP address and the ones'(1s) complement of the CIDR Subnet Mask.
  Disclaimer: In Python, every default integer variable is regarded as a signed integer. This means that if you do ones' complement of an signed integer, the language will regarded as the signed complement of that number (which is either its negative/positive complement number). In Python you can work around this by either working with the ctype library or by doing an additional bitwise operation onto the inverted CIDR Subnet Mask. In this implementation, I chose to do the later, by bitwise AND between the inverted CIDR Subnet Mask and the number 0xffffffff.
  ip | (~masks[cidr] & 0xffffffff) in line 48 in the code.

• Start(First) Usable Address of the Subnet This is calculated as: Start Address + 1

• End(Last) Usable Address of the Subnet This is calculated as: Broadcast Address - 1

• Total Hosts in Subnet
  This is calculated as: 2**(N [of bits in IPv4] - CIDR [prefix]) - 2 [Router and Broadcast IPs]