"Virtual LANs (VLANs) divide your local network logically,
rather than relying only on physical switches and routers. This
gives you a wonderful level of flexibility. Physically subnetting a
LAN requires multiple routers, or routers with multiple network
interfaces, and multiple switches. With VLANs you can segment your
network and make changes without being limited by hardware. You can
create intricate blends of both physical and logical subnets until
either your head explodes, or everything works happily.
"For example, frugal network administrators can run several
subnets off a single Ethernet switch and router. You need a switch
that supports VLANs to make this work, client network interfaces
require VLAN addresses, and you'll have to configure routing to
pass traffic between your new virtual subnets..."