A first information processor transmits a bubble packet to a second
communication control unit for leaving transmission record in a first
communication control unit by way of the first communication control
unit, a second information processor transmits a reply packet to one or
more ports including at least the bubble packet transmitting port as the
port of the first communication control unit used in transmission of
bubble packet, and the first information processor receives the reply
packet transmitted from the second information processor by way of the
second communication control unit. In this configuration, the invention
presents a communication system capable of establishing communication
between plural information processors for communicating by way of
communication control unit (NAT).