If you can solve this one, you get bonus points in my book.
We have a small satellite office in Colorado Springs. The office has a workstation that sometimes acts as an ad-hoc server, a Sharp printer, a couple of permanent laptops, and a whole bunch of guests who connect to a "VLAN" via a few UNIFI wireless access points. At the center of it all is a Cisco ASA 5505 that handles DHCP and connects to another 5505 in our Denver office over VPN/IPSec.
With an abundance of guests, the subnet mask on the Colorado Springs office was too small. My firewall guru hopped in and changed the subnet mask to /24 to accommodate the additional guests. During the reconfiguration of the ASA (which he did remotely), he repeatedly lost connectivity using the Cisco ASDM tool. (He could continue to connect via SSH, but not the ASDM tool). Suffice to say, he was finally able to change the subnet mask to /24 without breaking the ASDM connectivity, but the Colorado Springs Cisco was being flakey.
One (and only one) of the permanent laptops in the Colorado Springs office quit "seeing" the remainder of the office. It can see the internet and ping the LAN's gateway, but it cannot print, utilize VPN/IPSec, or ping any other IPs in the office other than the gateway. Additional details:
The laptop works fine in the Denver office. It prints and can even use VPN/IPSec going from Denver to Colorado Springs.
The other workstation and laptop in the Colorado Springs office have full LAN connectivity. Everything simply works.
I've alternately disabled the both the NIC and the wireless card on the laptop and have not seen a change in behavior.
I've completely turned of the firewall on the laptop and have not seen a change in behavior.
I checked the laptop for recently installed items. The only two installs that jibe with the faulty LAN behavior are a Office 2007 Compatibility Pack update and a .NET 4.5 Framework update. I have a hard time imagining either of these causing the issue, but I could be wrong about the .NET 4.5 Framework update.
I have a hard time imagining the changes to the firewall settings being responsible for this behavior, but the timing of the firewall reconfiguration and the laptop's sudden failure to see the Colorado Springs network does match.