nmap

nmap -sP 192.168.1.0 /24 (icmp ping the network see if anyone responds)

nmap -sP -PT80 192.168.1.0 /24 (see if port 80 responds)

nmap -P0 --top-ports 192.168.1.127-254 > file (scan a range top 10 ports only output to file)

nmap -sS 192.168.1.254

-sS (SYN scan)
-sT (TCP connect)
-sU (UDP port scans)

-O (detect OS)
-sV (service version, which version of sendmail is running on port 25)

-P0 (don't ping just scan)

-T (pre set timing options used to avoid IDS/IPS)

-p (choose ports)

-F (fast scan)
-n (dont do revers DNS lookup)