The
painful and
troublesome
process of moving a
TCP/IP network over to a new
range of
IP addresses.
Involves router configuration, creating new reverse DNS zone files, overhauling the old DNS zone files, and convincing InterNIC to grok where your nameservers are in the new set of addresses.
Oh, and then you have to actually tell the individual machines what their new addresses are. Unless you're using DHCP, which is a Good Idea.