I use NMAP to perform port scanning, e.g., TCP SYN scan:
nmap -sS 192.168.0.12.
I've two virtual machines running Windows XP and Windows 7, respetively. These have several services running, e.g., port 445.
However, if the Windows software-based firewall is enabled, then NMAP reports
filtered for all well-known ports, even thus they are in fact
open. If I disable the firewall, then NMAP reports all ports correctly.
Have I done anything wrong? What is the purpose of port scanning, if a simple software-based firewall makes it useless? Should a network administrator modify firewall rules to reflect real-world scenario, i.e., web server, before I would see the open ports in the result set?