If you're using a load balancer that obscures the remote client's true IP address, you can run this IHttpModule to take the first IP address from the X-Forwarded-For header, and overwrite the REMOTE_ADDR and REMOTE_HOST server variables. (We tried to use the URL Rewrite module that everyone normally recommends to do this, but it's buggy.)

ClientIP.cs

using System;
using System.Web;
using System.Text.RegularExpressions;

namespace HttpModules
{
        /// <summary>
        /// This module handles complications from our load balancer configuration not properly passing the client's true IP
        /// address to our code via the REMOTE_ADDR and REMOTE_HOST variables. We tried to use URL Rewrite to compensate for
        /// this, but it does not run when default documents are being accessed (a longstanding bug).
        ///
        /// Add a reference to this class to your <modules> section in either applicationHost.config (for your entire server),
        /// or the web.config of a specific web server. 
        /// </summary>
        public class ClientIP : IHttpModule
        {
                /// <summary>
                /// The IP we want will be in $1. X-Forwarded-For can carry multiple IPs in a comma-separated
                /// list, but the first IP should belong to the original client.
                /// </summary>
                private static Regex REGEX_FIRST_IP = new Regex(@"^\s*(\d+\.\d+\.\d+\.\d+)", RegexOptions.Compiled);

                /// <summary>
                /// This overwrites the REMOTE_ADDR and REMOTE_HOST server variables with the first IP taken from the
                /// X-Forwarded-For header, if it exists. Otherwise it does nothing.
                /// </summary>
                void OnBeginRequest(object sender, EventArgs e)
                {
                        HttpApplication app = (HttpApplication)sender;

                        string headervalue = app.Context.Request.Headers["X-Forwarded-For"];

                        if (headervalue != null)
                        {
                                Match m = REGEX_FIRST_IP.Match(headervalue);

                                if (m.Success)
                                {
                                        app.Context.Request.ServerVariables["REMOTE_ADDR"] = m.Groups[1].Value;
                                        app.Context.Request.ServerVariables["REMOTE_HOST"] = m.Groups[1].Value;
                                }
                        }
                }

                /// <summary>
                /// Put anything here that needs to happen when the module is disposed of.
                /// </summary>
                void IHttpModule.Dispose()
                {

                }

                /// <summary>
                /// This wires up our OnBeginRequest method to the BeginRequest event.
                /// </summary>
                /// <param name="app"></param>
                void IHttpModule.Init(HttpApplication app)
                {
                        app.BeginRequest += OnBeginRequest;
                }
        }
}