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
    result := StrPas(Hostent^.h_name)
  else
    result:='';
end;

//Пример использования
procedure TForm1.Button1Click(Sender: TObject);
begin
  Label1.Caption := IPAddrToName(Edit1.Text);
end;