Gerald is correct. By default network inputs assign the sending device/server's ip address as the host name, you can switch it so that Splunk will do a reverse DNS lookup on the IP and grab that as the host name. If you are using Splunk 4.1.x you can make this change in the Splunk Manager, previous versions require you to make the change directly to inputs.conf. (See the docs http://www.splunk.com/base/Documentation/latest/Admin/Monitornetworkports for details.)
Once you make the change as new data comes in the host name will appear in the summary view. However, since the host field is an indexed field your change will not be retroactive, your old data will still have IPs for host names. You can either just wait for the older data to age out of your system and the IP hosts will disappear, or you could delete the older data manually once it's lost it relevance.
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