Way back when I wrote about the “Observed IP Range” information reported for physical NICs in v5 hosts.
I noted among other things that the field seems to be populated by a process that runs at boot time or when you run the “Add Adapter Wizard”.
With 6.x, ESXi seems to either periodically check for Observed Networks, or at least it checks when you refresh the “Physical adapters” tab. This makes it little easier to double-check your network setup.
Note that it tends to post 0.0.0.1-255.255.255.254 if it detects traffic at all and only posts ranges corresponding to broadcast traffic the NICs receive.
There’s about a 30 second delays between starting the ping xx.xx.xx.255 -t (which works to populate the fields) and the networks updating.