DC is your domain. If you want to connect to the domain example.com than your dc's are DC=abc,DC=net.
You actually don't need any hostname or ip address of your domain controller.
Just imagine that you're connecting to the domain itself. So for connecting to the domain example.com you can simply write.
DirectoryEntry directoryEntry = new DirectoryEntry("LDAP://abc.net");
So
protected void LoginButton_Click(object sender, EventArgs e)
{
if (AuthenticateUser("LDAP://abc.net", txtLoginID.Text, txtPassword.Text))
{
System.Web.Security.FormsAuthentication.RedirectFromLoginPage(txtLoginID.Text, false);
Response.Redirect("~/home.aspx");
}
}