WORD software_checksum(BYTE *rxtx_buffer, WORD len, DWORD sum)
{
	// build the sum of 16bit words
	while(len>1)
	{
		sum += 0xFFFF & (*rxtx_buffer<<8|*(rxtx_buffer+1));
		rxtx_buffer+=2;
		len-=2;
	}
	// if there is a byte left then add it (padded with zero)
	if (len)
	{
		sum += (0xFF & *rxtx_buffer)<<8;
	}
	// now calculate the sum over the bytes in the sum
	// until the result is only 16bit long
	while (sum>>16)
	{
		sum = (sum & 0xFFFF)+(sum >> 16);
	}
	// build 1's complement:
	return( (WORD) sum ^ 0xFFFF);
}

// build the sum of 16bit words
	while(len>1)
	{
		sum += 0xFFFF & (*rxtx_buffer<<8|*(rxtx_buffer+1));
		rxtx_buffer+=2;
		len-=2;
	}

// if there is a byte left then add it (padded with zero)
	if (len)
	{
		sum += (0xFF & *rxtx_buffer)<<8;
	}

// now calculate the sum over the bytes in the sum
	// until the result is only 16bit long
	while (sum>>16)
	{
		sum = (sum & 0xFFFF)+(sum >> 16);
	}

// build 1's complement:
	return( (WORD) sum ^ 0xFFFF);

ตัวอย่างการคำนวณค่า checksum ของ IP Header.



   45 00 
+ 00 34 =  4534
+ 48 18 =  8d4c
+ 40 00 =  cd4c
+ 40 06 = 10d52
+ 00 00 = 10d52
+ 0a 01 = 11753
+ 01 4c = 1189f
+ 0a 01 = 122a0
+ 01 01 = 123a1

23a1 + 0001 = 23a2

นำ 23a2 มาทำ 1's complement = dc5d