PURPOSE: A method for calculating a checksum of a TCP segment/UDP data gram during IPv4(Internet Protocol version 4)/IPv6(Internet Protocol version 6) translations is provided to implement a required checksum value without including a value of a newly specified data area while calculating a checksum, thereby reducing a time for calculating the checksum value. CONSTITUTION: A TCP segment of an IPv4-based network is received through a receiving processor(S2001). A checksum reader reads a checksum within a TCP header, and a receiving pseudo header area checksum processor and a receiving header area checksum processor operate values of a pseudo header and a header area(S2002). A complement translator translates the read checksum to a complement of '1'(S2003). An adder adds the operated values of the pseudo header and the header area(S2004). A pseudo header area checksum processor and a header area checksum processor operate values of a pseudo header area and a header area of a packet, respectively(S2005-S2006). Another adder adds the operated values to the operated values of the receiving checksum processor, and translates the added value to '1' by another complement translator, to obtain a new checksum(S2007-S2008). A checksum insertion processor inserts the new checksum value to a newly translated TCP header, to transmit the value to an IPv6-based network(S2009).
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