NMAP
Nmap, short for "Network Mapper," is a widely-used open-source tool for network exploration and security auditing. It's designed to discover hosts and services on a computer network, thus creating a map of the network's structure. Nmap utilizes raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and numerous other characteristics. It's highly flexible, offering a range of scanning techniques, including TCP SYN scan, TCP connect scan, UDP scan, and more, making it invaluable for network administrators, security professionals, and hackers alike for network inventory, and detecting vulnerabilities.
Install
Debain based distributions
RHEL based distributions
Discover hosts on a network
Discover all open ports on a host
Without -p- nmap will only scan the top 1000 ports