Unix 几种网络地址

1.IPV4 internet address
struct in_addr {
  in_addr_t   s_addr;           /* 32-bit IPv4 address */
                                /* network byte ordered */
};

2.IPV4 socket address
struct sockaddr_in {
  uint8_t         sin_len;      /* length of structure (16) */
  sa_family_t     sin_family;   /* AF_INET */
  in_port_t       sin_port;     /* 16-bit TCP or UDP port number */
                                /* network byte ordered */
  struct in_addr  sin_addr;     /* 32-bit IPv4 address */
                                /* network byte ordered */
  char            sin_zero[8];  /* unused */
};


3.IPV6 internet address
struct in6_addr {
  uint8_t  s6_addr[16];          /* 128-bit IPv6 address */
                                 /* network byte ordered */
};

4.IPV6 socket address
struct sockaddr_in6 {
  uint8_t         sin6_len;      /* length of this struct (28) */
  sa_family_t     sin6_family;   /* AF_INET6 */
  in_port_t       sin6_port;     /* transport layer port# */
                                 /* network byte ordered */
  uint32_t        sin6_flowinfo; /* flow information, undefined */
  struct in6_addr sin6_addr;     /* IPv6 address */
                                 /* network byte ordered */
  uint32_t        sin6_scope_id; /* set of interfaces for a scope */
};


5.Generic socket address
struct sockaddr {
  uint8_t      sa_len;
  sa_family_t  sa_family;    /* address family: AF_xxx value */
  char         sa_data[14];  /* protocol-specific address */
};

6.New Generic socket address
struct sockaddr_storage {
  uint8_t      ss_len;       /* length of this struct (implementation dependent) */
  sa_family_t  ss_family;    /* address family: AF_xxx value */
  /* implementation-dependent elements to provide:
   * a) alignment sufficient to fulfill the alignment requirements of
   *    all socket address types that the system supports.
   * b) enough storage to hold any type of socket address that the
   *    system supports.
   */
};

注意到，所有的socket address的开头两个字段是uint8_t ss_len和sa_family_t ss_family，第一个字段指明数据结构的长度，可以用来定位地址结构，第二个指定地址类型，可以用来区别各种地址。正因为前两个字段一样，才能用sockaddr或者sockaddr_storage统一表示socket address，应用程序只需要将具体的地址类型(如sockaddr_in6或者sockaddr_in)显示的转换成通用地址类型(sockaddr或者sockaddr_storage)，内核网络模块就可定位地址数据结构并判断地址类型。现在的实现中，如connect,bind等，都采用sockaddr作为通用地址。

下面的图清晰的说明了各种网络地址的关系