Disclosed are a method for reporting a transmission state by an Internet protocol (IP) and a network system using the same. The method includes the steps of determining whether discard of any datagrams occurs, and if the discard of any datagram occurs, obtaining an address of a destination by parsing a discarded datagram, and transmitting an ICMP message for reporting an occurrence of the discard of the datagrams to the address of the destination. The network system includes a source for transmitting a datagram, a destination for receiving the datagram, and a router for performing routing between a network and the source or the destination.