Nmap is a utility for port scanning large networks, although it works fine for single hosts. Sometimes you need speed, other times you may need stealth. In some cases, bypassing firewalls may be required. Not to mention the fact that you may want to scan different protocols (UDP, TCP, ICMP, etc.). Nmap supports Vanilla TCP connect() scanning, TCP SYN (half open) scanning, TCP FIN, Xmas, or NULL (stealth) scanning, TCP ftp proxy (bounce attack) scanning, SYN/FIN scanning using IP fragments (bypasses some packet filters), TCP ACK and Window scanning, UDP raw ICMP port unreachable scanning, ICMP scanning (ping-sweep), TCP Ping scanning, Direct (non portmapper) RPC scanning, Remote OS Identification by TCP/IP Fingerprinting, and Reverse-ident scanning. Nmap also supports a number of performance and reliability features such as dynamic delay time calculations, packet timeout and retransmission, parallel port scanning, detection of down hosts via parallel pings.
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