Private VLANs are used to provide layer 2 isolation between members of the same broadcast domain. Private VLANs are documented in RFC 5517. In a standard VLAN environment traffic between members of the same VLAN can flow without restrictions. We can think of private VLANs like a segmentation of a normal VLAN in multiple subdomains. This feature is available only on layer 3 Catalyst 3560s and higher switches. Private VLANs can be used to address two issues found in service provider networks. First using normal VLANs an ISP must assign one VLAN per customer and thus a scalability problem would arise if the ISP needs to support more than 4094 clients which is the maximum number of supported VLANs by a device. Secondly when using IP routing each VLAN requires a separate subnet, which can lead to IP address management problems by wasting unused IP addresses.