TCP Scan (-sT)
In a TCP scan, the scanner sends TCP packets to the target server. This technique involves a full three-way handshake.

Port status:
If the port is open, the scanner sends a SYN flag. The server responds with a SYN-ACK flag, and the scanner completes the handshake by sending an ACK flag.

• If the port is closed, the server responds with an RST (Reset) flag, indicating that the connection cannot be established.

SYN Scan(-sS)
A SYN scan does not complete the full three-way handshake. It is often used to evade older Intrusion Detection Systems (IDS). While an IDS might block a full TCP scan, SYN scan can bypass IDS because it does not fully establish a connection.

Port status:
- If the port is open, the scanner sends a SYN flag. The server responds with a SYN-ACK flag, but instead of completing the handshake, the scanner sends an RST (Reset) flag to terminate the connection.

UDP Scan(-sU)
A UDP scan is connectionless, meaning it does not involve a three-way handshake like TCP scans.

Port status:
- If the port is open, the server does not send any response.
- If the port is closed, the server typically responds with an ICMP "Port Unreachable" message (not an RST flag, as RST is specific to TCP).

NULL , FIN and XMAS scan

These scanning technique used for to avoid firewall.

NULL scan(-sN)
It send the TCP packet with NULL flag.

FIN Scan(-sF)
it send the TCP packet with FIN flag

Xmas scan(-sX)
it send the tcp packet with multiple flag set like FIN,PUSH

Ping Scan (-sn)
A ping scan is a technique used to scan a range of IP addresses to check whether they are active or not.