You could consider the following:
Group devices by traffic patterns - Devices that communicate extensively between each other are good candidates to be grouped into a common VLAN.
Group devices for security - It is often a good practice to put servers and key infrastructure in their own VLAN, isolating them from the general broadcast traffic and enabling greater protection.
Group devices by traffic types - VoIP quality is improved by isolating VoIP devices to their own VLAN. Other traffic types may also warrant their own VLAN. Traffic types include network management traffic, IP multicast traffic such as video, file and print services, email, Internet browsing, database access, shared network applications, and traffic generated by peer-to-peer applications.
Group devices geographically - In a network with limited trunking, it may be beneficial to combine the devices in each location into their own VLAN.
It all depends on what the requirements are.
Routing can take care for users in Site A authenticating from a server in Site B if theyre on another subnet/vlan if needed.
hth
Bilal
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