Okay, this is a really short article, but I felt the need to write this because too many people seem to do it the wrong way.

Problem

First: getting the public IP address of a machine shouldn’t be done locally (Dns.GetHostAddresses(Dns.GetHostName()) <- this is evil).

Why?

This method is good only if you need the local IP address. If you need the public one consider that not every machine is directly connected to the internet. Some might be sitting behind a firewall or a router, but the framework won’t know it and will return an address which is valid only inside your network. In order to get the public IP address, which allows computers from the internet to communicate with you, you need to query some entity from outside your network.

Solution

One reliable way to solve this is to ask a website what’s your external IP address - there are many websites that can be used for this and some of them will even return your IP address as a plain text - which is nice.

Here’s a short example of how you should do it (don’t forget to include System.Net):

  • this snippet should only give you the idea of how to do this, don’t use it as it is.
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string getExternalIP()
{
    using (WebClient client = new WebClient())
    {
         return client.DownloadString("https://api.ipify.org/");
    }
}

What if the website is down?

Well, this is a risk that can be avoided by using…more websites :) and a bunch of try-catch blocks. If a website can’t be reached, we query the next one. Simple as that.

A more reliable example here:

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string getExternalIP()
{
    using (WebClient client = new WebClient())
    {

        try
        {
            return client.DownloadString("http://wtfismyip.com/text");
        }
        catch (WebException e) 
        {  
            // offline...
        }

        try
        {
            return client.DownloadString("https://api.ipify.org/");
        }
        catch (WebException e)
        {
            // offline too...
        }

        // if we got here, all the websites are down, which is unlikely
        return "Check internet connection?";
    }
}

It can still be improved by doing all the connections at the same time (putting each one in a different thread) - this will reduce the waiting time (if the first websites are down).

However these modifications would exceed the purpose of the article.