Understanding of internet verification algorithms

It takes almost two weeks to verify the InternetAlgorithmI still don't quite understand. The Reference URL is as follows:

Introduction to http://blog.csdn.net/namelcx/article/details/6866720 TCP/IP checksum algorithm.
Checksum algorithm of http://www.cnblogs.com/tuyile006/archive/2006/12/06/583647.html IP packet)
Http://blog.csdn.net/max2008/article/details/1720706 IP packet checksum algorithm.
Http://phinecos.blog.51cto.com/1941821/368159 C ++ exercises (17th) --- Internet checksum Algorithm
Calculation Method of checksum in http://blog.csdn.net/jiangnanyouzi/article/details/3333486 TCP/IP.

Few people can understand the truth. For thisCodeI don't know. I don't understand it. I'm stupid!

 
Unsigned short checksum (unsigned short * Buf, int nword) {unsigned long sum; For (sum = 0; nword> 0; nword --) sum + = * Buf ++; sum = (sum> 16) + (sum & 0 xFFFF); sum + = (sum> 16); Return ~ SUM ;}
 

Especially in the last three or two rows, do you understand? Please leave a message. Thank you for sharing it!

1) sum = (sum> 16) // calculate the 16-bit height of sum.
 

 
2) (sum & 0 xFFFF); // The sum is 16 bits.
 

 
(1) 2) the possible carry produced by the formula addition is composed of the lower formula. If there is no carry, the result of> 16 is zero, which has no effect on the result.
 

 
Sum + = (sum> 16 );
 

Every 16-bit binary anticode is added to this end, but I still do not understand the http://blog.csdn.net/jiangnanyouzi/article/details/3333486 in this great god mentioned.

Unsigned short a [10] = {0x4500, 0x059a, 0x82b9, 0X4000, 0x3206, 0x4f79, 0xa66f, 0x08ee, 0xc0a8, 0x0126 };

Question 1: The first 16-bit ox4500 is understandable. 4 is IPv4 and 5 is the header length. Taking the 32-bit binary number as a counting unit, it indicates five 32-bit characters. The rest will not be expressed. What is the basis for this expression?

Question 2: It seems that the final result is 0 xFFFF. If not, isn't it?