Many servers and network devices like routers and switches provides access control features like ACLs, IP Filters, firewall rules and so on, to give access to their services to specific IP addresses (usually the administrator's workstations).
The main purpose IRS is to find out the IP restrictions that have been set for a particular service on a host. It combines "ARP Poisoning" and "Half-Scan" techniques and tries totally spoofed TCP connections to the selected port of the target. IRS is no a port scanner but a valid IP source scanner for a given service.
