Ndiff is a tool to aid in the comparison of Nmap
scans. Specifically, it takes two Nmap XML output files and prints the
differences between them: hosts coming up and down, ports becoming
open or closed, and things like that. Ndiff can produce output in
human-readable text or machine-readable XML formats. Many people like
to scan their networks regularly (daily, weekly, etc.) and then use
ndiff to easily detect any changes. The scans, ndiff run, and emailed
report are often automated using tools such as cron on UNIX or the
Scheduled Tasks tool on Windows. Ndiff is also used by
the Zenmap GUI to compare scan
Ndiff is integrated with Nmap versions 4.85BETA1 and later, which you
can obtain from the Nmap download page.
The best Ndiff documentation is currently the Ndiff man page.