Backtrack has its Nmap installed inside. Nmap ("Network Mapper") is a free and open source (license) utility for network discovery and security auditing. Nmap is Flexible, Powerful, Portable, Easy, Free, Well, Supported, Acclaimed, and Popular.
Nmap has some basic options:
-sT : TCP Connect, using method connect(), we can determine wheter the port opened or not, but it is easily to detect by firewall/intrusion detection software.
-sS : SYN stealth scan, using syn to begin connection then sending rst to stop initiation connection, so that no connection will be made. it results in harder detection.
-sF, -sN, -sX : FIN, NULL, Xmass Tree Scan. FIN will send Fin Flag, Null will set no flag and Xmass Tree sets the FIN, URG and PUSH flags. It is used to avoid system detection.
-sP : Ping Scan , scan with ping command to determine whether the target online or not
-sU : UDP Scan , scan port with UDP packet.
-sO : IP Protocol Scan, scan port using ip raw packet.
-sI : Idle Scanning, more advanced scanning mode
-sV : Version Detection,
-sA : ACK Scan,-sW, -sR, -sL : Window Scan, RPC Scan, List Scan