You might be more satisfied with a lookup table for ip_address to hostname. This can be done easily with a CSV hosts file, or be tied into DNS lookups automatically. See http://www.splunk.com/base/Documentation/latest/Knowledge/Addfieldsfromexternaldatasources for information on setting either a static or dynamic lookup.
Something like this should work:
your_1st_search | join type=left ip_address [search your_2nd_search | fields ip_address hostname]