ASP.NET MVC 4 Custom Validation for CIDR Subnet

I recently worked on a rWhois project that required writing an ASP.NET MVC 4 based admin portal. One of the last things I tackled was adding validation to inputs. Adding validation to a MVC model is super-simple with Data Annotations. By adding these annotations to your model, both client and server-side validation comes to life. There’s a great tutorial describing how to accomplish this on the site:

The in-box DataAnnotations class provides numerous built-in validation attributes that can be applied to any property. Things like strings of specific length, or integers within a range, or even regex matching is supported using the built-in validation attributes – and they are automatically wired up when you use Visual Studio to add views with scaffolding. But what about when the built-in attributes don’t quite do what you need?

I needed to create validation that confirms the input is both in proper CIDR format (ie. and not “some text”) and that the supplied CIDR subnet is valid (ie. is a proper network/bitmask, but is not). Validating that the input is in the proper format can easily be done with regex. Just add the following to annotation to the appropriate property on your model:

[RegularExpression(@"^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])(\/(\d|[1-2]\d|3[0-2]))$", ErrorMessage="Not valid CIDR format")]

To check that the supplied input is a valid network/bitmask CIDR combination, I needed custom validation. You can easily extend the ValidationAttribute and implement IClientValidatable to add your custom validation. First, you’ll want to create a new class and reference the DataAnnotations and Mvc namespaces:

using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;
namespace MvcApplication1.Validation
    public class CIDRSubnet : ValidationAttribute, IClientValidatable

Next, we’ll want to create the appropriate server-side validation by overriding the ValidationResult class and adding the CIDR subnet check:

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        //validate that valus is valid cdir
        string[] cidr = ((string)value).Split(new char[] { '/' });
        string[] ip = cidr[0].Split(new char[] { '.' });
        int i = (Convert.ToInt32(ip[0]) << 24) | (Convert.ToInt32(ip[1]) << 16) | (Convert.ToInt32(ip[2]) << 8) | (Convert.ToInt32(ip[3]));
        int mask = Convert.ToInt32(cidr[1]) == 0 ? 0 : ~((1 << (32-29)) - 1);
        int network = i & mask;
        if (i != network) return new ValidationResult("Not a valid CIDR Subnet");
        return ValidationResult.Success;

Now we want to create our client side validation function. Create a new javascript file and make sure it’s referenced in your View:

function ipv4checker(input) {
    //confirm cidr is valid network
    var cidr = input.split('/');
    var base = cidr[0];
    var bits = cidr[1];
    var ip = base.split('.');
    var i = (ip[0] << 24 | ip[1] << 16 | ip[2] << 8 | ip[3]);
    var mask = bits == 0 ? 0 : ((1 << (32 - bits)) - 1) ^ 0xFFFFFFFF;
    var network = i & mask;
    if (i == network) { return true; }
    else { return false; }

Assuming jquery and jquery unobtrusive validation libraries have already been added to your project, you can then add a method to the validator, and wire it up. I’m not passing any parameters here, but you could also include parameters:

//custom vlidation rule - check IPv4
    function (value, element, param) {
    return ipv4checker(value);
//wire up the unobtrusive validation
    ("checkcidr", function (options) {
        options.rules["checkcidr"] = true;
        options.messages["checkcidr"] = options.message;

Then, you’ll need to implement the GetClientValidationRules method in your custom validation class:

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
        ModelClientValidationRule rule = new ModelClientValidationRule();
        rule.ValidationType = "checkcidr";
        rule.ErrorMessage = "Not a valid CIDR Subnet";
        return new List<ModelClientValidationRule> { rule };

Lastly, add a reference to your custom validation class in the model and add the custom validation attribute to your property:

using System.ComponentModel.DataAnnotations;
using MvcApplication1.Validation;
namespace MvcApplication1.Models
    public class Allocation
        [RegularExpression(@"^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])(\/(\d|[1-2]\d|3[0-2]))$", ErrorMessage="Not valid CIDR format")]
        [CIDRSubnet(ErrorMessage="Not a valid CIDR Subnet")]
        public string IPNetwork { get; set; }

One thought on “ASP.NET MVC 4 Custom Validation for CIDR Subnet

  1. Pingback: Add Custom Validation for ASP.NET MVC 4 CIDR Subnet | Windows Hosting Done Right – Cytanium

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