本文摘自 linux kernel ip.h,感谢开源的GNU

struct ip

  {

#if __BYTE_ORDER == __LITTLE_ENDIAN

    unsigned int ip_hl:4;		/* header length */

    unsigned int ip_v:4;		/* version */

#endif

#if __BYTE_ORDER == __BIG_ENDIAN

    unsigned int ip_v:4;		/* version */

    unsigned int ip_hl:4;		/* header length */

#endif

    u_int8_t ip_tos;			/* type of service */

    u_short ip_len;			/* total length */

    u_short ip_id;			/* identification */

    u_short ip_off;			/* fragment offset field */

#define	IP_RF 0x8000			/* reserved fragment flag */

#define	IP_DF 0x4000			/* dont fragment flag */

#define	IP_MF 0x2000			/* more fragments flag */

#define	IP_OFFMASK 0x1fff		/* mask for fragmenting bits */

    u_int8_t ip_ttl;			/* time to live */

    u_int8_t ip_p;			/* protocol */

    u_short ip_sum;			/* checksum */

    struct in_addr ip_src, ip_dst;	/* source and dest address */

  };

struct timestamp

  {

    u_int8_t len;

    u_int8_t ptr;

#if __BYTE_ORDER == __LITTLE_ENDIAN

    unsigned int flags:4;

    unsigned int overflow:4;

#elif __BYTE_ORDER == __BIG_ENDIAN

    unsigned int overflow:4;

    unsigned int flags:4;

#else

# error	"Please fix <bits/endian.h>"

#endif

    u_int32_t data[9];

  };

struct iphdr

  {

#if __BYTE_ORDER == __LITTLE_ENDIAN

    unsigned int ihl:4;

    unsigned int version:4;

#elif __BYTE_ORDER == __BIG_ENDIAN

    unsigned int version:4;

    unsigned int ihl:4;

#else

# error	"Please fix <bits/endian.h>"

#endif

    u_int8_t tos;

    u_int16_t tot_len;

    u_int16_t id;

    u_int16_t frag_off;

    u_int8_t ttl;

    u_int8_t protocol;

    u_int16_t check;

    u_int32_t saddr;

    u_int32_t daddr;

    /*The options start here. */

  };

/*

 * Time stamp option structure.

 */

struct ip_timestamp

  {

    u_int8_t ipt_code;			/* IPOPT_TS */

    u_int8_t ipt_len;			/* size of structure (variable) */

    u_int8_t ipt_ptr;			/* index of current entry */

#if __BYTE_ORDER == __LITTLE_ENDIAN

    unsigned int ipt_flg:4;		/* flags, see below */

    unsigned int ipt_oflw:4;		/* overflow counter */

#endif

#if __BYTE_ORDER == __BIG_ENDIAN

    unsigned int ipt_oflw:4;		/* overflow counter */

    unsigned int ipt_flg:4;		/* flags, see below */

#endif

    u_int32_t data[9];

  };