How to get a user’s client IP address in ASP.NET?

Often you will want to know the IP address of someone visiting your website. While ASP.NET has several ways to do this one of the best ways we’ve seen is by using the “HTTP_X_FORWARDED_FOR” of the ServerVariables collection.

Here’s why…

Sometimes your visitors are behind either a proxy server or a router and the standard Request.UserHostAddress only captures the IP address of the proxy server or router. When this is the case the user’s IP address is then stored in the server variable (“HTTP_X_FORWARDED_FOR”).

So what we want to do is first check “HTTP_X_FORWARDED_FOR” and if that is empty we then simply return ServerVariables("REMOTE_ADDR").

While this method is not foolproof, it can lead to better results. Below is the ASP.NET code in VB.NET, taken from James Crowley’s blog post “Gotcha: HTTP_X_FORWARDED_FOR returns multiple IP addresses”

C#

protected string GetIPAddress()
{
    System.Web.HttpContext context = System.Web.HttpContext.Current; 
    string ipAddress = context.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];

    if (!string.IsNullOrEmpty(ipAddress))
    {
        string[] addresses = ipAddress.Split(',');
        if (addresses.Length != 0)
        {
            return addresses[0];
        }
    }

    return context.Request.ServerVariables["REMOTE_ADDR"];
}

VB.NET

Public Shared Function GetIPAddress() As String
    Dim context As System.Web.HttpContext = System.Web.HttpContext.Current
    Dim sIPAddress As String = context.Request.ServerVariables("HTTP_X_FORWARDED_FOR")
    If String.IsNullOrEmpty(sIPAddress) Then
        Return context.Request.ServerVariables("REMOTE_ADDR")
    Else
        Dim ipArray As String() = sIPAddress.Split(New [Char]() {","c})
        Return ipArray(0)
    End If
End Function