If you connect to a website through a wifi network where there are a number of access points with the same ESSID and a different BSSID, would the servers of that website know which AP was used? So, for example, if 2 AP "A" and "B" had the same ESSID and a different BSSID, would the packet transmitted to the server contain the information on whether AP "A" or AP "B" was used? If they don't, what is the latest node where that information can be found?
It depends what is behind those access points.
If they are all on one flat Ethernet network then only the access points and switches will know where the client is (and the switches will only know which of their own switchports it is behind which may or may not represent a single AP).
If the are on different VLANs with seperate IP subnets and no NAT then you can work out from the IP address which access point the client is on.
If there are NATs involved things get more complex still.
Using the WLC, this is something that can easily be figured out through IP and MAC address logging.
I would hope that you would have multiple VLAns in that type of environment or at least three in a small one, like MGMT, corp and guest at the very least....
In that scenario you would have three logical IP blocks and this would provide you a lot of advantages from traffic flow to security.