Guides

The ideas behind the tools, explained with worked examples. Every number on these pages comes from the same code the calculators run.

• IPv4 subnetting explained

  CIDR notation, subnet masks, usable hosts and how to split a network — with a table of every common prefix.

• Reserved IP addresses in cloud subnets

  How many addresses AWS, Azure, Google Cloud and Oracle Cloud keep in each subnet, and which ones.

• Pixel density explained

  What PPI measures, how to calculate it, and what it means for sharpness on monitors, laptops and phones.

• PC energy cost and power supply efficiency

  80 PLUS ratings, power drawn from the wall, what a PC costs to run a year, and when a more efficient PSU pays for itself.