If I have 2 airport extremes in bridge mode, will my network be limited to 50 concurrent users, or will that limit be dependent on my router that is serving dhcp?
The maximum number of users is a limitation imposed by your router.
The AirPort will handle up to 50 wireless users per device, (and sometimes 51), so if you have two AirPorts, you can in theory have a total of 100 wireless users, providing that your router is able to issue IP addresses to all of them.
Realistically, you really would not want to put more than 20-25 users on an AirPort, since things really slow down about this point.
If I have 2 airport extremes in bridge mode, will my network be limited to 50 concurrent users, or will that limit be dependent on my router that is serving dhcp?
