Many businesses have networks setup by whoever happened to volunteer or the employee most annoyed that their wifi didn’t work. Typically the setup involves 1-5 pieces of consumer grade network hardware, mostly switches or wireless access points. There isn’t anything wrong with this if things are configured correctly . The problem is, consumer grade equipment is usually configured out of the box to act as the only piece of network hardware on the LAN. Consumer grade stuff typically has everything enabled out of the box and doesn’t work well with other devices.
Cause of all problems-Everything is a router
Most consumer network devices are configured to act as their own router. Typically the device will have NAT and DHCP on by default. This is by far the biggest cause of problems.
Result: multiple subnets
Because everything has NAT enabled, and directions typically tell you to connect the “internet” side to the port labeled WAN, most consumers will end up with multiple subnets on their LAN. What typically results is a network where wifi and wired devices cannot talk to each other, or certain PC’s cannot see a printer or network drive. The easiest way to fix this is to avoid connecting anything to WAN ports on any devices except the one directly connected to the internet. Increasingly, modems are doing their own NAT, so it might be better to avoid using the WAN port at all. I typically put a piece of tape over the WAN port and write NO! on installations where users are likely to experiment. It has saved many headaches.
Result: DHCP issues
Again due to default configuration, most with networks like this will have machines that can’t talk to each other, random disconnect issues, and OS connection warnings like “multiple devices using the same IP” conflicts or the more vague “network connectivity limited”. The solution to this is to login to every access point, router, and routing capable device(almost anything these days) and disable DHCP, save for one. You need to make sure there is a single device on the network giving out IP addresses, and that every permanent device which isn’t, like an access point or printer, has a static ip address outside the range of the ones the DHCP server sends out. Since modems increasingly come with DHCP enabled, the easiest solution is to just disable DHCP on everything else, since for DNS reasons it makes sense to have your DHCP server on the modem.
Result: Multiple Wifi networks or roaming issues
A properly setup wifi network will only have one name, and users will be able to move between them with no interruption. Frequently, access points are configured to either be on separate subnets or have different names. Fixing the subnet issue is usually as easy as not using the WAN port for anything, and as long as the wifi username and password are the same on both access points, roaming works magically.