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unit Unit1;

interface

uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls, Winsock;

const
  iphlpapilib = 'iphlpapi.dll';

type
  TForm1 = class(TForm)
    Label1: TLabel;
    Button1: TButton;
    Edit1: TEdit;
    procedure Button1Click(Sender: TObject);
  private
   { Private declarations }
  public
    { Public declarations }
  end;

function SendArp(ipaddr:ulong;
  temp:dword;
  ulmacaddr:pointer;
  ulmacaddrleng:pointer) : DWord; StdCall;
var
  Form1: TForm1;
implementation

  {$R *.DFM}
function SendARP; external iphlpapilib name 'SendARP';

function GetMacAddr(const IPAddress: string): string;
var
  lDestIP:ulong;
  lMacAddr:array[0..5] of byte;
  lPhyAddrLen:ulong;
begin
  lDestIP:=inet_addr(PansiChar(IPAddress));
  lPhyAddrLen:=length(lMacAddr);
  sendarp(lDestIP,0,@lMacAddr, @lPhyAddrLen);
  Result:=Format('%2.2x-%2.2x-%2.2x-%2.2x-%2.2x-%2.2x',
      [Integer(lMacAddr[0]), Integer(lMacAddr[1]),
      Integer(lMacAddr[2]), Integer(lMacAddr[3]),
      Integer(lMacAddr[4]), Integer(lMacAddr[5])]);
end;


procedure TForm1.Button1Click(Sender: TObject);
begin
  label1.caption:=GetMacAddr(Edit1.Text);
end;

end.                    

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function SendARP(const DestIP, SrcIP: in_addr; pMacAddr: PULONG; var PhyAddrLen: ULONG): DWORD; stdcall; external IPHLPAPILIB name 'SendARP';

function GetMacAddr(const IPv4: String): String;
  const
    SrcIP: in_addr = (S_addr: 0);
  var
    DestIP: in_addr;
    MacAddr: array[0..5] of Byte;
    MacAddrLen, RC: DWORD;
    i: Integer;
begin
  Result := '';
  DestIP.S_addr := inet_addr(PChar(IPv4));
  MacAddrLen := Length(MacAddr);
  // http://msdn.microsoft.com/en-us/library/aa366358(VS.85).aspx
  RC := SendARP(DestIP,SrcIP,@MacAddr,MacAddrLen);
  case RC of
    NO_ERROR:
      begin
        if (MacAddrLen = 6) then
          for i := 0 to 5 do begin
            if (i > 0) then
              Result := Result +'-';
            Result := Result +IntToHex(MacAddr[i],2);
          end
        else
          Result := 'Error: MacAddrLen='+IntToStr(MacAddrLen);
      end;
    ERROR_GEN_FAILURE:
      Result := 'ERROR_GEN_FAILURE';
    ERROR_INVALID_PARAMETER:
      Result := 'ERROR_INVALID_PARAMETER';
    ERROR_INVALID_USER_BUFFER:
      Result := 'ERROR_INVALID_USER_BUFFER';
    ERROR_BAD_NET_NAME:
      Result := 'ERROR_BAD_NET_NAME';
    ERROR_BUFFER_OVERFLOW:
      Result := 'ERROR_BUFFER_OVERFLOW';
    ERROR_NOT_FOUND:
      Result := 'ERROR_NOT_FOUND';
    else
      Result := 'Error: RC='+IntToStr(RC);
  end;
end;                    

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DestIP.S_addr := inet_addr(PAnsichar(AnsiString(IPv4)));