uses 
  WinSock; 

function LocalIP: string; 
type 
  TaPInAddr = array[0..10] of PInAddr; 
  PaPInAddr = ^TaPInAddr; 
var 
  phe: PHostEnt; 
  pptr: PaPInAddr; 
  Buffer: array[0..63] of Char; 
  I: Integer; 
  GInitData: TWSAData; 
begin 
  WSAStartup($101, GInitData); 
  Result := ''; 
  GetHostName(Buffer, SizeOf(Buffer)); 
  phe := GetHostByName(buffer); 
  if phe = nil then Exit; 
  pPtr := PaPInAddr(phe^.h_addr_list); 
  I := 0; 
  while pPtr^[I] <> nil do 
  begin 
    Result := inet_ntoa(pptr^[I]^); 
    Inc(I); 
  end; 
  WSACleanup; 
end; 

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; 

procedure TForm1.Button1Click(Sender: TObject); 
begin 
  ShowMessage(IPAddrToName(LocalIp)); 
end; 

{**************************************} 

// Function to get the IP Address from a Host 

function GetIPFromHost(const HostName: string): string; 
type 
  TaPInAddr = array[0..10] of PInAddr; 
  PaPInAddr = ^TaPInAddr; 
var 
  phe: PHostEnt; 
  pptr: PaPInAddr; 
  i: Integer; 
  GInitData: TWSAData; 
begin 
  WSAStartup($101, GInitData); 
  Result := ''; 
  phe := GetHostByName(PChar(HostName)); 
  if phe = nil then Exit; 
  pPtr := PaPInAddr(phe^.h_addr_list); 
  i := 0; 
  while pPtr^[i] <> nil do 
  begin 
    Result := inet_ntoa(pptr^[i]^); 
    Inc(i); 
  end; 
  WSACleanup; 
end;