masscan with the right setup (namely hardware + drivers but also connection obviously) can scan the entire IPv4 space (+ all ports) in ~5 minutes.

Source Code: https://github.com/robertdavidgraham/masscan

Article from PoC || GTFO with more internal details on how it works: https://www.alchemistowl.org/pocorgtfo/pocorgtfo15.pdf (Page 66) [Note: PDF is both a valid PDF + valid ZIP file with source code]