There are 4 sets of Locally Administered Address Ranges that can be used on your network without fear of conflict, assuming no one else has assigned these on your network:
x2-xx-xx-xx-xx-xx x6-xx-xx-xx-xx-xx xA-xx-xx-xx-xx-xx xE-xx-xx-xx-xx-xx
Replacing x with any hex value.
See http://en.wikipedia.org/wiki/MAC_address for more information.
Update 2014-10-27: one my reader notify me that some MAC OUI comply with rules above are used by some vendors. See example below:
Therefore I would always recommend to validate it at
For example 02-00-00 is not used by anybody so you can most probably use it for internal purpose.
Private network (internal)
Private network (service provider - subscriber)