IPアドレスからホスト名を得る
struct HOSTENT* FAR gethostbyaddr(
const char* addr,
int len,
int type
);
----
addr
-A pointer to an address in network byte order. 

len
-The length of the address, which must be 4 for PF_INET addresses. 

type
-The type of the address, which must be PF_INET.