API编程部分：http://www.cnblogs.com/Jimmy1988/p/7895213.html

1. 协议简介

此处，我们主要介绍Linux编程常用的三种协议(TCP/UDP/IP), 关于三种协议的定义，可参见各自的头文件：

• /usr/include/linux/tcp.h
• /usr/include/linux/udp.h
• /usr/include/linux/ip.h

①. TCP

/** Come from /usr/include/linux/tcp*/
struct tcphdr {__be16  source;__be16  dest;__be32  seq;__be32  ack_seq;
#if defined(__LITTLE_ENDIAN_BITFIELD)__u16   res1:4,doff:4,fin:1,syn:1,rst:1,psh:1,ack:1,urg:1,ece:1,cwr:1;
#elif defined(__BIG_ENDIAN_BITFIELD)__u16   doff:4,res1:4,cwr:1,ece:1,urg:1,ack:1,psh:1,rst:1,syn:1,fin:1;
#else
#error  "Adjust your <asm/byteorder.h> defines"
#endif__be16  window;__sum16 check;__be16  urg_ptr;
};

②. UDP

/** Come from /usr/include/linux/udp.h*/
struct udphdr {__be16  source;__be16  dest;__be16  len;__sum16 check;
};

③. IP

/* * Comes from /usr/include/linux/ip.h*/struct iphdr {
#if defined(__LITTLE_ENDIAN_BITFIELD)__u8    ihl:4,version:4;
#elif defined (__BIG_ENDIAN_BITFIELD)__u8    version:4,ihl:4;
#else
#error  "Please fix <asm/byteorder.h>"
#endif__u8    tos;__be16  tot_len;__be16  id;__be16  frag_off;__u8    ttl;__u8    protocol;__sum16 check;__be32  saddr;__be32  daddr;/*The options start here. */
};

2. IPv4相关操作

/* Internet address.  */
typedef uint32_t in_addr_t;
struct in_addr
{in_addr_t s_addr;
};

①. IPv4地址转换

②. 获取ID

3. 大端小端

• 大端：Big-Endian
  即内存高地址存放数据的低字节：如0x1234，存放在0x4000~0x4001，则0x4000存0x12,0x4001存0x34
  网络字节顺序统一采用大端
• 小端：Little-Endian
  与大端相反，即高地址存放高字节
  x86系列处理器为小端模式

①.程序判断大小端

#include <stdio.h>int main()
{union end_un{unsigned short int word;char ch;}endian;endian.word = 0x1234;if(endian.ch == 0x12){printf("This is Big-Endian!\n");}else if(endian.ch == 0x34){printf("This is Little-Endian!\n");}return 0;
}

②. 字节顺序转化函数

头文件：#include <arpa/inet.h>