nmap is probably taking a long time to perform DNS resolution, or perhaps timing out on hosts for which it cannot perform such resolution. Try running nmap with the '-n' option - see the text below, which was pasted from the nmap man page:
-n (No DNS resolution)
Tells Nmap to never do reverse DNS resolution on the active IP addresses it finds. Since DNS
can be slow even with Nmap’s built-in parallel stub resolver, this option can slash scanning
Last edited by HughA; 20th January 2008 at 04:41 AM.