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Nmap Tutorial: Common Commands: Page 2 of 3

 

Scan a range of IPs

This is the command to scan a range of IPs. Scanning a range of IPs is useful when trying to determine where a network attack may be occurring. Being able to scan multiple IPs also saves valuable time when tracing a network attack:

nmap 192.168.0.1-20

 

Scan a subnet

This command scans a subnet. Scanning a subnet will allow the scan to monitor multiple hosts. This command is useful when checking on multiple networks as well:

nmap 192.168.0.1/24

 

Nmap port selection

To utilize Nmap effectively, you will need to understand how to use the port selection options. The port selection options determine what ports will be scanned and whether the scan order is random or in a sequential order.

Scan a single port

This is the command to scan a single port. Some malware will consistently operate on a specific port on every host it infects. By knowing these ports, you can sometimes quickly determine what kind of malware you are dealing with. A single port scan would be useful in this situation:

nmap -p 80 192.168.0.9

 

Scan a range of ports

This is the command to scan a range of ports 1-100. The versatility of this command allows you to focus on specific ranges of ports:

nmap -p 1-100 192.168.0.9

 

Scan 100 most common ports (fast)

These are a number of different default scans. -f will scan the most common 100 ports used:

nmap -f 192.168.0.9

 

The preceding is the command to scan the most common ports. Some common examples would be ports 20, 21, 23, 25, and 53, to name a few. This is known as a fast scan.

Scan all 65535 ports

This is the command to scan all ports. There are a total of 65,535 ports. A hacker will not usually employ this type of scan. Instead most hackers will initially use a scanning technique known as half-open scanning. The scan all ports command is better utilized by a threat hunter monitoring the network:

nmap -p- 192.168.0.9

 

Nmap port scan types

There are many different types of port scan that can be used with Nmap. It is important to know which type of port scan to use depending on your objective. For example, if you want to determine which TCP ports are active on a targeted host, run a TCP port scan. Hackers will often use various port scans to see if they can find a vulnerable open port to use as an attack vector.

Scan using TCP SYN scan (default)

This command determines whether the port is listening. Using this command is a technique called half-open scanning. It is called half-open scanning because you don't establish a full TCP connection. Instead, you only send a SYN packet and wait for the response. If you receive a SYN/ACK response that means the port is listening:

nmap -sS 192.168.1.1

 

Scan using TCP connect

This is the command to scan using the TCP connect option. If a user does not have raw packet privileges, this is the command they will use:

nmap -sT 192.168.0.9

 

Privileged access is necessary to perform the default SYN scans. If privileges are not sufficient, a TCP connect scan will be used. A TCP connect scan needs a full TCP connection to be established, and is known to be a slower scan than SYN scans. Disregarding discovery is often required as many firewalls or hosts will not answer to ping, so it could be missed, unless you choose the -Pn parameter. Of course, this can make the scan times much longer as you could end up sending scan probes to hosts that are not even there.

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