The first parameter to add is the type of scan, and there are many. The most common are -sS for Syn scan and -sT for a Connect scan. A syn scan is also called a half open scan because it sends a syn packet, and if it receives a syn-ack, it then tears down the session without completing the TCP three way handshake.
If you are not root, the default is a Connect scan. A Syn scan requires root privileges. A connect scan is slower and noisier because establishes a full session .
UDP scanning is very slow because most UDP services don’t send a response, unless it’s DNS, SNMP or some other service that interacts with the source. UDP scanning relies on an ICMP port unreachable message. Some networks filter these I=CMP packets even though this is not good networking practice and can cause issues on the network. Use -sU to conduct a UDP scan.
So a Syn scan would be: nmap -sS 126.96.36.199 and a Connect scan would be nmap -sT 188.8.131.52
To specify what ports to scan, use -p. For example, you can use a comma separated list like -p 80,443,8080,3128. To specify a range of ports, you can use -p 1023-2056 which would scan all ports beginning with 1023 thru 2056 inclusively. You can also use CIDR notation and a combination of different types.
*** This is a Security Bloggers Network syndicated blog from JeffSoh on NetSec authored by JeffSoh. Read the original post at: https://jeffsoh.blogspot.com/2020/02/nmap-for-beginners-part-2.html