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

procedure TForm1.Button1Click(Sender: TObject);

begin

 Label1.Caption:=IPAddrToName(Edit1.Text);

end;