sentences:  Some aggregators (for example, Bloglines and Google Reader) include a number of unique users on whose behalf the feed is being downloaded in the HTTP request.Other readers -- such as web browsers -- can be counted by noting the number of unique IP addresses that retrieve the file in a given period.This provides an estimate of actual readership, although it is probably higher than the real number because people may sign up for accounts with multiple aggregators and never delete their subscriptions and because they may read the same feeds at different computers, or the same computer may have a different IP address at different times.