function SendARP(ipaddr: ulong; temp: dword; ulmacaddr: pointer; ulmacaddrleng:

    pointer): dword; stdcall; external 'Iphlpapi.dll' Name 'SendARP';

 

function IP2Mac(ipaddr:ULong):String;

var

  AMac: array [0 .. 5] of BYTE;

  l: ulong;

  r: integer;

begin

  l:=6;  

  r := SendARP(ipaddr, 0, @AMac, @l);

  if r=0 then

    Result:=Format('%2.2x-%2.2x-%2.2x-%2.2x-%2.2x-%2.2x', [AMac[0], AMac[1], AMac[2], AMac[3], AMac[4],AMac[5]])

  else

    Result:='';

end;

来源：http://blog.qdac.cc/?p=2902