The Sessions by Country Report tabulates the number of unique sessions in the given time frame grouped by the country where the request originated.  This information is determined from the country code of the host name (if present).  This report requires that the web server log files contain IP addresses that are resolved to host names.  If the log file does not contain domain host names, then the results of those IP address not resolved will be listed as unavailable.  Please refer to your web server documentation for more information on resolving IP addresses to domain host names.