NMAP command to check whether port is open or close
Most of the time when there is a problem connecting to any specific port we use telnet command to verify, before that you can use NMAP command to verify whether port is open or close.
NMAP abbreviation is network mapper
NMAP is used to scan ports on a machine, either local or remote machine (just you require ip/hostname to scan).
NMAP is can be installed on windows, Sun Solaris machines too.
NMAP can be used to scan large networks, remember I am saying large networks.
Please do not try to use NMAP on machines which you don’t have permission.
Example1 : Using NMAP in normal way, i.e. to scan a particular system for open ports
#nmap hostname
Example2 : Scanning for a single port on a machine
#nmap –p 22 hostname
This will scan for 22 port is open on a host or not. And here –p indicates port.
Example3 : For scanning only ports
#nmap –F hostname
-F is for fast scan and this will not do any other scanning like IP address, hostname, operating system, and uptime etc. It’s very much fast as it said in man pages.
Example4 : For scanning only TCP ports
#nmap –sT hostname
Here s is for scanning and T is for only scanning of TCP ports
Example5 : For scanning only UDP ports
#nmap –sU hostname
Here U indicates UDP port scanning
Exmaple6 : Scanning for ports and to get what is the version of different services running on that machine
#nmap –sV hostname
V indicates version of each network service running on that host
Example7 : To check which protocol is supported by the remote machine
#nmap –sO hostname
Example8 : To scan a system for operating system and uptime details
# nmap -O hostname
-O is for operating system scan along with default port scan
Example9 : Scanning a network
#nmap networkID/subnetmask
For the above command you can try in this way
#nmap 192.168.0.0/24
