Increasing Ip address Scope

Our network is now at the point where we are regularly running out of free IP addresses.

We currently use an address range of 172.21.1.0 and have a subnet of 255.255.255.0

I was wondering if its possible to easily reconfigure the network to give us some more addresses.

is there an easy way to do this? if there's not an easy way, then is there a hard way and is it worth doing? are there any decent tutorials on the net that explain the process?

Thanks!