In addition to the above-mentioned problems with FlexLM we run three "redundant license servers". You'd think this would increase reliability, but it doesn't. The first time you discover this is when all three can't talk to each other. They get the brilliant idea that each should become a "master" server. When the network comes back up nobody is willing to become a "slave" server again (which pretty much mirrors human life), so your licenses don't work because you need at least one master and one slave for them to work.

Mention this to the creator of FlexLM (as we did, several times, in frustration) and its falling upon deaf ears. The only solution is to take down two of the servers then bring them back up again. Where's the increased reliability in that? Oh, I almost forgot: the network connectivity usually breaks during weekends and off-hours, so you have the machines page you when this happens. Live close by or learn to remote login and reboot machines.