Html.TextBoxFor does not show updated value in POST action

In my view I have

      <%:Html.LabelFor(model => model.IPAddress)%>

    <div class="editor-field">
        <%:Html.TextBoxFor(model => model.IPAddress)%>
        <%:Html.ValidationMessageFor(model => model.IPAddress)%>
    </div>

In my controller(post method), I have this

[HttpPost]
public ActionResult Manipulation(MyModel model){
  //I change modele here
  if(something)
    model.IPAddress="100.100.100.100";
  return View(model);
}

So, my question is: When I change model, the TextBoxFor does not change his value. TextBoxFor get his value when I get from get method to the post, and later I cannot change value of TextBoxFor. I debug, and my model have new value, but TextBoxFor does not show new value.

Can you help me?