Mapper is a connectivity tester. Like when you ping a host to see if it is up, if the host sends an echo reply back then it is alive. With security awareness heightening pinging a host is becoming less effective due to firewalling. Mapper uses other "features" in TCP/IP to gain responses out of hosts.
It will test using the following techniques:
* TcpSyn
* TcpAck
* TcpSynAck
* TcpFin
* TcpNull
* TcpXmas
* IcmpEcho