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Nmap features include: Fast scan (nmap -F [target]) – Performing a basic port scan for fast result. Host discovery – Identifying hosts on a network. For example, listing the hosts that respond to TCP and/or ICMP requests or have a particular port open. Port scanning – Enumerating the open ports on target hosts.
(Port zero is not a usable port number.) Most services use one, or at most a limited range of, port numbers. Some port scanners scan only the most common port numbers, or ports most commonly associated with vulnerable services, on a given host. The result of a scan on a port is usually generalized into one of three categories:
The port numbers in the range from 0 to 1023 (0 to 2 10 − 1) are the well-known ports or system ports. [3] They are used by system processes that provide widely used types of network services. On Unix-like operating systems, a process must execute with superuser privileges to be able to bind a network socket to an IP address using one of the ...
Some port scanners scan only the most common port numbers, or ports most commonly associated with vulnerable services, on a given host. See: List of TCP and UDP port numbers. The result of a scan on a port is usually generalized into one of three categories: Open or Accepted: The host sent a reply indicating that a service is listening on the port.
Network enumeration is a computing activity in which usernames and info on groups, shares, and services of networked computers are retrieved. It should not be confused with network mapping, which only retrieves information about which servers are connected to a specific network and what operating system runs on them.
This is different from a port sweep that will only identify open ports, which are assumed to be associated with the default service for that port. The difference is that a port scan and a port sweep will detect that a device has a port open and would assume that the port is associated with the service normally associated with that port.
Some examples of service ports used for banner grabbing are those used by Hyper Text Transfer Protocol , File Transfer Protocol , and Simple Mail Transfer Protocol ; ports 80, 21, and 587 respectively. Tools commonly used to perform banner grabbing are Telnet, Nmap and Netcat.
Port scanning, and nmap, may help to identify which ports are open on suspect IPs, however, even when it says that proxy ports are open the default scan does not check to see if it is an open proxy using that port. It could be a closed proxy expecting authentication, or even a normal website.