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How To Use Nmap to Scan Open Ports?

by | Mar 11, 2021

Don’t know how to use Nmap to scan Open Ports? We can help you.

A single IP address can have several services running, such as a web server, an application server, etc. In order for each of them to communicate, they listen and communicate on a specific port.

Hence, when we make a connection to a server, we connect to both the IP address and a port.

For example, when we connect to, we connect to the server on port 443, the default port for secure web traffic.

As part of our Server Management Services, we assist our customers with several Nmap queries.

Today, let us see how to use Nmap to scan Open Ports.


How To Use Nmap to Scan Open Ports?

In this article, let us discuss ports in detail.

World’s leading port security network scanner, Nmap hosted security tool can help us determine how well the firewall and security configuration is working.

“nmap” scans IPv4 addresses by default. However, if we specify the proper option, it can also scan IPv6 addresses.


Identifying Common Ports

Ports are specified by a number ranging from 1 to 65535.

We cannot register ports between 49152 and 65535 and are suggested for private use.

Since there is a vast number of ports available, we don’t have to bother the majority of the services that tend to bind to specific ports.

However, there are some ports that are worth knowing due to their ubiquity. The following are a few:

  • 20: FTP data
  • 21: FTP control port
  • 22: SSH
  • 23: Telnet
  • 25: SMTP
  • 43: WHOIS protocol
  • 53: DNS services
  • 67: DHCP server port
  • 68: DHCP client port
  • 80: HTTP – Unencrypted Web traffic
  • 110: POP3 mail port
  • 113: Ident authentication services on IRC networks
  • 143: IMAP mail port
  • 161: SNMP
  • 194: IRC
  • 389: LDAP port
  • 443: HTTPS – Secure web traffic
  • 587: SMTP – message submission port
  • 631: CUPS printing daemon port
  • 666: DOOM – This legacy game actually has its own special port

To configure a specific application we need to find the appropriate ports.

We should make sure, both the client and server are configured to use a non-standard port.

To get a list of a few common ports, look at the /etc/services file:

$ less /etc/services

The output will have a list of common ports and their associated services:

. . .
tcpmux 1/tcp # TCP port service multiplexer
echo 7/tcp
echo 7/udp
discard 9/tcp sink null
discard 9/udp sink null
systat 11/tcp users
daytime 13/tcp
daytime 13/udp
netstat 15/tcp
qotd 17/tcp quote
msp 18/tcp # message send protocol
. . .


Checking Open Ports

Generally, we have a number of tools to scan for open ports. netstat is the default for most Linux distributions.

To easily discover the services running, we can issue:

$ sudo netstat -plunt

We will receive a result like the following:

Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0* LISTEN 785/sshd
tcp6 0 0 :::22 :::* LISTEN 785/sshd

Our output shows the port and listening socket associates with the service and lists both UDP and TCP protocols.


Using Nmap

Part of securing a network involves infiltrating the network and discovering weaknesses in the same way an attacker might.

Out of all of the available tools for this, the most powerful is the Nmap tool.

To install Nmap on an Ubuntu or Debian machine, we run:

$ sudo apt-get update
$ sudo apt-get install nmap

The installation gives us an improved port mapping file. A more extensive association between ports and services can be seen at:

$ less /usr/share/nmap/nmap-services
. . .
tcpmux 1/tcp 0.001995 # TCP Port Service Multiplexer [rfc-1078]
tcpmux 1/udp 0.001236 # TCP Port Service Multiplexer
compressnet 2/tcp 0.000013 # Management Utility
compressnet 2/udp 0.001845 # Management Utility
compressnet 3/tcp 0.001242 # Compression Process
compressnet 3/udp 0.001532 # Compression Process
unknown 4/tcp 0.000477
rje 5/udp 0.000593 # Remote Job Entry
unknown 6/tcp 0.000502
echo 7/tcp 0.004855
echo 7/udp 0.024679
echo 7/sctp 0.000000
. . .


Scanning Ports with Nmap

Nmap can reveal a lot of information about a host. For this reason, our Support Techs recommend testing it on your own servers or after notifying the owners.

The Nmap creators provide a test server located at This or your own servers are good targets for practicing Nmap.

Moving ahead let us see a few common operations we can perform with Nmap.

Scan for the host operating system:

$ sudo nmap -O

We assume the host is online and skip the network discovery portion. This helps if we get the reply, “Note: Host seems down” in other tests.

Add this to the other options:

$ sudo nmap -PN

Scan without performing a reverse DNS lookup on the IP address specified. This will speed up the results in most cases:

$ sudo nmap -n

Scan a specific port instead of all common ports:

$ sudo nmap -p 80

To scan for TCP connections, Nmap can perform a 3-way handshake, with the targeted port. Execute it like this:

$ sudo nmap -sT

Similarly, to scan for UDP connections, we type:

$ sudo nmap -sU

We can scan for every TCP and UDP open port using:

$ sudo nmap -n -PN -sT -sU -p-

A TCP “SYN” scan exploits the way that TCP establishes a connection.

To perform an SYN scan, we execute:
$ sudo nmap -sS

A more stealthy approach is to send invalid TCP headers. This will work on non-Windows-based servers.

We can use the “-sF”, “-sX”, or “-sN” flags. All of which will produce the response we look for:

$ sudo nmap -PN -p 80 -sN

To check the version of a service is running on the host, we try this command. It tries to determine the service and version by testing different responses from the server:

$ sudo nmap -PN -p 80 -sV

Finally, we can use Nmap to scan multiple machines.

In addition, to specify a range of IP addresses with “-” or “/24” to scan a number of hosts at once, we run:

$ sudo nmap -PN

Or scan a network range for available services with a command like this:

$ sudo nmap -sP

This should help us explore the networking vulnerabilities.

[Need help with Nmap? We are here for you]



To conclude, the world’s leading port security network scanner, Nmap can determine how well the security configuration is working. Today we learned from our Support Techs how to use Nmap to Scan Open Ports.


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