sure you can its called CIDR ( Classless inter-Domain Routing) or Supernetting. Its just like Sub Netting But you go the other way for a more detailed explination you can look and any CCNP (Sybex or Cisco) BSCI ( Building Scaleable Cisco Internetworks) book or try this link.
http://www.firewall.cx/supernetting-chart.php