A private vlan lets you control access between groups of ports within a VLAN. Chances are you do not need to use a private VLAN, but here are the basics.
There are 3 types of ports within a private VLAN.
Isolated-Ports that are in an isolated VLAN cannot communicate with any other port in the VLAN EXCEPT a promiscuous port(s) to which the ISOLATED vlan is mapped.
Community-Community VLANs are created to allow only certain ports within the VLAN to communicate. Any ports in the same community within the same Private VLAN can communicate freely with one another, they can also communicate with the promiscuous port(s) that are mapped to the community VLAN.
Promiscuous port-A port that belongs to the primary VLAN which can communicate with isolated and community VLANs that are directly mapped to the port.
Basically the outside world sees only the Primary Private VLAN. The isolated and community vlans are secondary (sub-) vlans within the primary VLAN. This allows you to control access between the ports, but still allows the devices to be in the same subnet. Private VLANs are useful for security between servers in a WEB hosting environment and they are also useful in DMZ's. The following links explain Private VLANs in greater detail.
http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008013565f.shtml
http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a0080094830.shtml
For specific Private VLAN configuration information on the 4500 check the following link:
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat4000/12_1_20/config/pvlans.htm
