ASP.NET- On the Master Page, the contents overlap with the header

On the Master Page, the contents overlap with the header.

I don't see what the problem is.
look at the picture and code below.

<form id="form1" runat="server">       

    <div class="navbar navbar-inverse navbar-fixed-top">
        <div class="container">
            <div class="navbar-header">
                <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
                    <span class="icon-bar"></span>
                    <span class="icon-bar"></span>
                    <span class="icon-bar"></span>
                </button>
                <a class="navbar-brand" runat="server" href="~/">Resu Service</a>
            </div>
            <div class="navbar-collapse collapse">
                <ul class="nav navbar-nav">
                    <li><a runat="server" href="~/">정보</a></li>
                    <li><a runat="server" href="~/">블로그</a></li>
                </ul>

            </div>
        </div>
    </div>
    <div>
        <asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
        </asp:ContentPlaceHolder>
    </div>
</form>