JavaScript: HTML Form - IP address validation

IP address validation

Every computer connected to the Internet is identified by a unique four-part string, known as its Internet Protocol (IP) address. An IP address consists of four numbers (each between 0 and 255) separated by periods. The format of an IP address is a 32-bit numeric address written as four decimal numbers (called octets) separated by periods; each number can be written as 0 to 255 (e.g., to

Example of valid IP address


Example of invalid IP address

  • 210.110 – must have 4 octets
  • 255 – must have 4 octets
  • y.y.y.y – the only digit has allowed
  • 255.0.0.y – the only digit has allowed
  • 666.10.10.20 – digit must between [0-255]
  • 4444.11.11.11 – digit must between [0-255]
  • 33.3333.33.3 – digit must between [0-255]

JavaScript code to validate an IP address

function ValidateIPaddress(ipaddress) 
 if (/^(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$/.test(myForm.emailAddr.value))
    return (true)
alert("You have entered an invalid IP address!")
return (false)

Explanation of the said Regular expression (IP address)

Regular Expression Pattern :

Character Description
/ .. / All regular expressions start and end with forward slashes.
^ Matches the beginning of the string or line.
25[0-5] Matches 250 or 251 or 252 or 253 or 254 or 255.
| or
2[0-4][0-9] Start with 2, follow a single character between 0-4 and again a single character between 0-9.
| or
? Matches the previous character 0 or 1 time.
[0-9][0-9] Matches a single character between 0-9 and again a single character between 0-9.
? Matches the previous character 0 or 1 time.
\. Matches the character "." literally.

Note: Last two parts of the regular expression is similar to above.

Syntax diagram - IP-address validation:

Syntax diagram - IP-address validation

Let apply the above JavaScript function in an HTML form.


<!DOCTYPE html>
<html lang="en">
<meta charset="utf-8">
<title>JavaScript form validation - checking IP address/title>
<link rel='stylesheet' href='form-style.css' type='text/css' />
<body onload='document.form1.text1.focus()'>
<div class="mail">
<h2>Input an IP address and Submit</h2>
<form name="form1" action="#"> 
<li><input type='text' name='text1'/></li>
<li class="submit"><input type="submit" name="submit" value="Submit" onclick="ValidateIPaddress(document.form1.text1)"/></li>
<script src="ipaddress-validation.js"></script>

JavaScript Code

function ValidateIPaddress(inputText)
 var ipformat = /^(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$/;
 return true;
 alert("You have entered an invalid IP address!");
return false; } }


Flowchart : JavaScript - ipaddress validation

CSS Code

li {list-style-type: none;
font-size: 16pt;
.mail {
margin: auto;
padding-top: 10px;
padding-bottom: 10px;
width: 400px;
background : #D8F1F8;
border: 1px soild silver;
.mail h2 {
margin-left: 38px;
input {
font-size: 20pt;
input:focus, textarea:focus{
background-color: lightyellow;
input submit {
font-size: 12pt;
.rq {
color: #FF0000;
font-size: 10pt;

View the Javascript IP address validation in the browser

file_download Download the validation code from here.

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JavaScript: Tips of the Day

Returns the first defined, non-null argument


const tips_coalesce = (...args) => args.find(v => ![undefined, null].includes(v));
console.log(tips_coalesce(null, undefined, '', NaN, 'Waldo'));