Multiple license manager servers can be installed on the same network. Each license manager controls access to a certain number of licenses on the network. Each ArcGIS installation chooses which license manager server to use.
For example, ArcInfo may be installed and managed from Server A while ArcSDE is installed and managed from Server B.
If one license server goes down, users can then point to another valid server. Windows users change servers using the Desktop Administrator. UNIX users change their ARCHOME or SDEHOME variable to point to a different server.
Separate license files are required, one for each license manager server. The SERVER line in each file is specific to a particular server. On Windows, each license server will also require a hardware key.
A multiple-server network is beneficial if you have servers that need to be rebooted or periodically removed from the network. You will need to have an adequate number of licenses on each server to support your users.
A workstation taken off the network needs to run a local license manager. See What is a standalone workstation for more information.