uses WinSock;
 
function GetIpAddressByName(const AComputerName: string): string;
var
  TMPResult: string;
  WSA: TWSAData;
  H: PHostEnt;
  P: PChar;
begin
  if WSAStartUp($101, WSA) = 0 then
    begin
      GetMem(P,255 + 1);
      StrPCopy(P, AComputerName);
      H:=GetHostByName(P);
      FreeMem(P);
      if H <> nil then
        begin
          P:=inet_ntoa(PInAddr(H^.h_addr_list^)^);
          TMPResult:=StrPas(P)
        end;
      WSACleanUp;
      if TMPResult <> '' then
        Result:=TMPResult
      else
          Result:= '0';
    end;
end;
 
 
procedure TForm1.Button1Click(Sender: Tobject);
begin
  Caption:=GetIpAddressByName('localhost');
end;