Nmap discovers hosts on a network primarily through a process called "host discovery," which involves sending specific types of packets to target IP addresses and analyzing the responses. Here are the key methods Nmap uses for host discovery:
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Ping Scans: Nmap can send ICMP Echo Request packets (similar to the ping command) to determine if a host is online. If the host responds with an ICMP Echo Reply, it is considered active.
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ARP Requests: On local networks, Nmap can use Address Resolution Protocol (ARP) requests to discover hosts. Since ARP operates at the data link layer, it can identify live hosts even if they do not respond to ICMP requests.
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TCP SYN Scans: Nmap can send TCP SYN packets to specific ports on the target hosts. If a host responds with a SYN-ACK packet, it indicates that the host is online and the port is open.
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TCP ACK Scans: By sending TCP ACK packets, Nmap can determine if a host is reachable. If a host responds with a RST packet, it indicates that the host is online.
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Service Probes: Nmap can also probe for specific services running on hosts to confirm their presence and gather additional information.
These methods can be combined or customized using various Nmap options to optimize the discovery process based on the network environment.
