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uses WinSock;

function IPAddrToName(IPAddr: Ansistring): string;
var SockAddrIn: TSockAddrIn;HostEnt: PHostEnt;WSAData: TWSAData;
begin
WSAStartup($101, WSAData);
SockAddrIn.sin_addr.s_addr := inet_addr(PAnsiChar(IPAddr));
HostEnt := gethostbyaddr(@SockAddrIn.sin_addr.S_addr, 4, AF_INET);
if HostEnt <> nil then Result := StrPas(Hostent^.h_name) else Result := '';
end;                    

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uses
  WinSock;

implementation

var
  WSAData: TWSAData;

function IPv4ToName(const IPv4ddn: AnsiString): String;
  var
    IPv4: in_addr;
    HostEnt: PHostEnt;
begin
  IPv4.S_addr := inet_addr(PAnsiChar(IPv4ddn));
  HostEnt := gethostbyaddr(@IPv4, SizeOf(IPv4), AF_INET);
  if Assigned(HostEnt) then
    Result := StrPas(Hostent^.h_name)
  else
    Result := '';
end;

initialization
  WSAStartup($101, WSAData); // der RC ist hoffentlich <> 0

finalization
  WSACleanup;