<p>To provide a communication device which can optimally decide a NAT characteristic of a repeater (router). In a communication device (100), a communication control unit (203) generates a NAT check request packet whose destination address is formed with one of IP addresses and one of port numbers of a server (104). A NAT check request transmission unit (204) transmits a NAT check request packet to the server. A NAT check response reception unit (202) receives a NAT check response packet including a transmission source IP address and a transmission source port number of the NAT check request packet received by the server as a return of the NAT check response packet. A relay characteristic judgment unit (201) judges a type of the NAT characteristic of the repeater according to the transmission source IP address and the transmission source port number of the NAT check request packet included in the NAT check response packet.</p>
