<style>
.a3{width:30;border:0;text-align:center}
</style>
<script>
function checkIPValid_1()
{
    if(event.shiftKey)
    {
        event.returnValue=false;
        return;
    }
    var keyCode = parseInt(event.keyCode);
    var result=true;
    if((keyCode==8) || (keyCode==37) || (keyCode==39) || (keyCode==9)) return;

    if((keyCode>=48) && (keyCode<=57)) return;
    else result=false;
    if((keyCode>=96) && (keyCode<=105)) return;
    else result=false;

    if((keyCode==110) || (keyCode==190) || (keyCode==32)) 
    {
        event.keyCode=9;
        return;
    }
    if(result==false) event.returnValue=false;
}
function checkIPValid_2(mask)
{
    
    if(typeof(mask) != 'object'){
        mask = eval(mask)
    }
    var maskInt=parseInt(mask.value);
    if((maskInt<0) || (maskInt>255))
    {
        mask.value=255;
        alert(maskInt+"不是个有效IP地址，请指定一个介于0和255之间的数值");
    }
}
</script>
IP地址输入
<div style="border-width:1;border-color:balck;border-style:solid;width:165;font-size:9pt">
<input name="ip1" maxlength="3" class="a3" onKeyDown="checkIPValid_1()" onKeyUp="checkIPValid_2(this)"/>.
<input name="ip2" maxlength="3" class="a3" onKeyDown="checkIPValid_1()" onKeyUp="checkIPValid_2(this)"/>.
<input name="ip3" maxlength="3" class="a3" onKeyDown="checkIPValid_1()" onKeyUp="checkIPValid_2(this)"/>.
<input name="ip4" maxlength="3" class="a3" onKeyDown="checkIPValid_1()" onKeyUp="checkIPValid_2(this)"/>
</div>