Converting IPv4 to decimal, and back

07 Sep 2011 — code

After reading this post by mrhinkydink I thought I’d share a c# way to accomplish the same thing; converting an IP address between dotted decimal notation and its numeric format. There are pre-written libraries and name spaces available everywhere to accomplish this task, but I’m going to show you the raw maths of the conversion process.

First, let’s construct a method to accept a string representation of an ipv4 address in dotted decimal notation (that’s 127.0.0.1) and convert it into numeric, or “long” format (2,130,706,433):

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public Double IpStringToLong( String ipString )
{
    var octets = ipString.Split('.');
    
    return Double.Parse( octets[3] ) 
         + Double.Parse( octets[2] ) * 256 
         + Double.Parse( octets[1] ) * 65536 
         + Double.Parse( octets[0] ) * 16777216;
}

Okay, and back the other way:

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public string LongToString(Double ipAsLong)
{
    var ip = String.Empty;

    for (var n = 1; n < 5; n++)
    {
        var octet = Convert.ToInt32(Math.Truncate(ipAsLong / Math.Pow(256, 4 - n)));
        ipAsLong = ipAsLong - (octet * Math.Pow(256, 4 - n));

        if (octet > 255) return "0.0.0.0";

        if (n == 1)
        {
            ip = octet.ToString(CultureInfo.InvariantCulture);
        }
        else
        {
            ip += "." + octet;
        }
    }
    return ip;
}

Finally if the input is either a string- or a long and we don’t know which but we want the opposite:

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var value = "127.0.0.1";
var value = 2130706433;

if (value.IndexOf(".") != -1)
{
    return String.Format("{0} in decimal is {1}", value, StringToLong(value));
}
return String.Format("{0} in dotted decimal is {1}", value, LongToString(value));

Edit 2018-02-21

Adding extension method class to this post:

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using System;
using System.Net;

namespace Program
{
    public static class IPAddressExtensions
    {
        public static uint ToInt(this IPAddress ip)
        {
            var octets = ip.ToString().Split('.');

            return uint.Parse(octets[3]) + 
                   uint.Parse(octets[2]) * 256 + 
                   uint.Parse(octets[1]) * 65536 + 
                   uint.Parse(octets[0]) * 16777216;
        }

        public static IPAddress ToIpAddress(this uint value)
        {
            return ToIpAddress((double)value);
        }

        public static IPAddress ToIpAddress(this double value)
        {
            var ip = string.Empty;

            for (var n = 1; n < 5; n++)
            {
                var octet = Math.Truncate(value / Math.Pow(256, 4 - n));

                value = value - octet * Math.Pow(256, 4 - n);

                if (octet > 255) return IPAddress.Parse("0.0.0.0");
                if (n == 1)
                {
                    ip = octet.ToString();
                }
                else
                {
                    ip += "." + octet;
                }
            }

            return IPAddress.Parse(ip);
        }
    }
}
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