[KERNEL]
<linux/in.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;
        __u16   tot_len;
        __u16   id;
        __u16   frag_off;
        __u8    ttl;
        __u8    protocol;
        __u16   check;
        __u32   saddr;
        __u32   daddr;
        /*The options start here. */
};

/* XXX: RFC2292bis */
struct ipv6hdr {
#if defined(__LITTLE_ENDIAN_BITFIELD)
        __u8                    tclass1:4,
                                version:4;
#elif defined(__BIG_ENDIAN_BITFIELD)
        __u8                    version:4,
                                tclass1:4;
#else
#error  "Please fix <asm/byteorder.h>"
#endif
        __u8                    tclass2_flow[3];

        __u16                   payload_len;
        __u8                    nexthdr;
        __u8                    hop_limit;

        struct  in6_addr        saddr;
        struct  in6_addr        daddr;
};


[void *  のかわり]
<sys/socket.h>
struct sockaddr {
       uin8_t		sa_len;			 /* linux で見あたらず。*/
       unsigned short	sa_family;		 /* アドレスファミリ*/
       char		sa_data[MAXSOCKADDRDATA];
};

[UNIX]

<sys/socket.h>
<sys/un.h>
struct sockaddr_un {
       unsigned short sun_family;
       cahr sun_path[UNIX_PATH_MAX];
};

[IPv4]
struct in_addr {
       in_addr_t s_addr;	/* IPv4 address (network byte order)*/
};

<netinet/in.h>
struct sockaddr_in  {
       uint8_t	      sin_len;		/* 構造体の大きさ(16バイト) linux で見あたらず。*/
       sa_family_t    sin_family;       /* AF_INET */
       in_port_t      sin_port;         /* 16ビット */
       struct in_addr sin_addr;         /* IPv4 address (network byte order)*/
       char	    sin_zero[8];	/* 未使用 */
};

[IPv6]
<netinet/in.h>
struct in6_addr {
       uint8_t  s6_addr[16];  // IPv6 address  
};

struct sockaddr_in6 {
       uint8_t           sin6_len;      // length of this struct
       sa_family_t       sin6_family;   //  AF_INET6
       in_port_t         sin6_port;     //  port number
       uint32_t          sin6_flowinfo; //  IPv6 flow infomation
       struct in6_addr   sin6_addr;     //  IPv6 address
       uint32_t	         sin6_scope_id; //  scope id
};

[HOST]
<netdb.h>
struct hostent {
       char *h_name;
       char **h_aliases;
       int h_addrtype;
       int h_length;
       char **h_addr_list;
};

hostent{}
+-------------+
| h_name    ------->"公式ホスト名"
+-------------+
| h_aliases ---------------->+-------------+
+-------------+              |           --------> "エイリアス#1"
| h_addrtype  |AF_INET[6]    +-------------+
+-------------+              =             =
| h_length    |4/16          +-------------+
+-------------+              | NULL        |
| h_addr_list -------+       +-------------+
+-------------+      |
                     |
		     +------>+-------------+
                             |           --------> in[6]_addr{}
			     +-------------+
			     =             =
			     +-------------+
			     | NULL        |
			     +-------------+

[ワイルドカードアドレス]
<netinet/in.h>に "INADDR_*" で存在。

[Multi Cast]
struct ip_mreq {
       struct in_addr imr_multiaddr; /* IPv4 class D multi cast address */
       struct in_addr imr_interface; /* IPv4 address of local interface */
};

struct ipv6_mreq {
       struct in6_addr ipv6mr_multiaddr; /* IPv6 multi cast address */
       unsigned int    ipv6mr_interface; /* interface index ofr 0 */
};

-------------------------------------------------------------------------------
  (TCP Server)      (TCP Client)       (UDP Server)      (UDP Client)
  +-----------+     +-----------+      +-----------+     +-----------+      
  | socket(2) |     | socket(2) |      | socket(2) |     | socket(2) |
  +-----------+     +-----------+      +-----------+     +-----------+      
      ||                ||                 ||                ||
      ||sockaddr_in     || sockaddr_in     ||                ||
      \/                \/                 \/                ||
  +-----------+     +-----------+      +-----------+         ||
  | bind(2)   |     | connect(2)|      | bind(2)   |         ||
  +-----------+     +-----------+      +-----------+         ||
      ||                ||                 ||                ||
      ||                ||                 ||                ||
      \/                ||                 \/                \/
  +-----------+         ||             +------------+    +-------------+
  | listen(2) |         ||             | recvfrom(2)|<==>| sendto(2)   |
  +-----------+         ||             | /sendto(2) |    | /recvfrom(2)|
      ||                ||             +------------+    +-------------+
      ||                ||                 ||		     ||         
      \/                ||                 ||		     ||         
  +-----------+         ||                 \/		     \/         
  | accept(2) |         ||             +-----------+     +----------+   
  +-----------+         ||             | close (2) |	 | close(2) |   
      ||                ||             +-----------+	 +----------+   
      ||                ||
      \/	        \/
  +-----------+    +-----------+
  | read(2)   |<==>| write(2)  |
  |  /write(2)|    |  /read(2) |
  +-----------+    +-----------+
      ||                ||        
      ||                ||        
      \/                \/        
  +-----------+    +-----------+
  | close(2)  |    | close(2)  |
  +-----------+    +-----------+