Locally Administered Address Ranges

MAC Addresses
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:

  02-07-01   (hex) RACAL-DATACOM

  02-1C-7C   (hex) PERQ SYSTEMS CORPORATION

  02-60-86   (hex) LOGIC REPLACEMENT TECH. LTD.

  02-60-8C   (hex) 3COM CORPORATION

  02-70-01   (hex) RACAL-DATACOM

  02-70-B0   (hex) M/A-COM INC. COMPANIES

  02-70-B3   (hex) DATA RECALL LTD

  02-9D-8E   (hex) CARDIAC RECORDERS INC.

  02-AA-3C   (hex) OLIVETTI TELECOMM SPA (OLTECO)

  02-BB-01   (hex) OCTOTHORPE CORP.

  02-C0-8C   (hex) 3COM CORPORATION

  02-CF-1C   (hex) COMMUNICATION MACHINERY CORP.

  02-E6-D3   (hex) NIXDORF COMPUTER CORPORATION

Therefore I would always recommend to validate it at
http://en.wikipedia.org/wiki/MAC_address#Bit-reversed_notation
For example 02-00-00 is not used by anybody so you can most probably use it for internal purpose.

IP Addresses
Private network (internal)
10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

Private network (service provider - subscriber)
100.64.0.0/10

See http://en.wikipedia.org/wiki/Reserved_IP_addresses for more information.