port scanning

Use netcat to check if UDP port 53 is open at IP 10.0.2.3

nc -vz -u 10.0.2.3 53

Use natcat to check if TCP ports 2000 to 2010 are open at IP 192.168.2.100

nc -zv 192.168.2.100 2000-2010

Use nmap with TCP connect option to check if TCP ports 2000 to 3000 are open at IP 10.1.1.34

nmap -sT -p 2000-3000 10.1.1.34

Same as above but skip nmap's host discovery (in case the target host is blocking ping probes)

nmap -sT -p 2000-3000 10.1.1.34 -Pn

Use nmap with SYN scan option (the default) to check if TCP port 22 is open at IP 10.1.1.70

nmap -sS -p 22 10.0.2.15

Use nmap to check if UDP ports 53,161 and 162 are open at IP 192.168.1.1

nmap -sU -p 53,161,162 192.168.1.1