Use nmap correctly

Question

I currently have a VPS in a data center, I asked the admin to open a port to install a ftp server. It opened port 3345, at first I tought the port is not forwarded correctly, because I ran nmap as this :

nmap 194.110.xxx.xx

Starting Nmap 6.00 ( http://nmap.org ) at 2013-12-17 14:33 EET
Nmap scan report for host-194-110-xxx-xx(194.110.xxx.xx)
Host is up (0.0022s latency).
Not shown: 995 closed ports
PORT     STATE    SERVICE
21/tcp   open     ftp
80/tcp   open     http
443/tcp  filtered https
3306/tcp open     mysql
3389/tcp open     ms-wbt-server

But when trying to connect with the client it works ok. Why nmap is not reporting that the port 3345 is open ?

Thank you

Answer 1

nmap by default only scans a subset of ports, called well known ports, this is to improve scan speed. 3345 is not one of the ones scanned in a default scan, therefore if you want to scan that port you will need to specify it using the -p switch, as in nmap -p T:3345

That's if you just want to use NMAP to scan that TCP port only. If you want to scan multiple ports you can scan using ranges, such as -p 1-1024, T:3345, 3000-5000.