IP address suddenly lost

Why would a workstation suddenly lose it's IP address?  I have had two Win7Pro workstations lose their IP addresses the last couple of days. Why would they do that?  The LAN adapter showed "unidentified network" and when I opened the IPv4 Properties window, it was set to "Obtain IP automatically", but all the cells were blank. The only thing we had done to either of these machines was to just restart them. When they came up, they had not obtained their IP's.  What should I check, the SBS server? The router? Or, are these two machines just being funky, suddenly? I decided to change to manual IP and entered all the IP, Subnet mask, default gateway and DNS server addresses and the workstation is back on the LAN.
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Ugo MenaCommented:
Depends on your DHCP server.

The lease may have expired and forced the clients to renew their IP addresses. Did your DHCP server get restarted? Probably a weird coincidence or a problem with the DHCP server's settings.

Some DHCP servers will keep the current leases in a cache to keep from handing out an IP already in use.

You can avoid this by setting IP manually (as you have done) or you can setup Static IPs within your DHCP server. This requires entering in the MAC address for each of the client machines and then assigning them a specific IP with the DHCP server.
Do those PC's by any chance have VirtualBox 5 installed? I had a similar issues on PC's where I upgraded from VBox 4.3 to 5. At first things worked fine, then after a while my IP's were gone, and later even the complete LAN went down, even for those PC's that didn't have VBox 5 installed. I then returned those PC's to VBox 4.3 and everything is fine again.
wfcrrAuthor Commented:
The only thing I can think of is Double Take.  We used Vision Solutions Double Take MOVE for windows to migrate the SBS server to new hardware on Sunday. When the new server wasn't working right, I took it off the LAN and rebooted the original server.  I wonder if that affected the two workstations?  The first one I had a problem with was Monday morning, could not reach it and had to drive to the location to figure out it had lost it's IP.  The issue with the second workstation occurred today when we rebooted it, it came up with no IP and same thing, had to go to the location and manually set IP.

I checked the SBS server and DHCP is running. Not sure how to see if a lease expired.  What should I check?
Since it works with static IP check your services on the workstation and make sure dhcp client service set to automatic. To check DHCP values you can open an cmd line and type in ipconfig /all
and get the dhcp assigned ip info.  If client svc running check DHCP server make sure it has IP's available and then check leases for any bad dhcp leases (you can also restart just the dhcp service to see if that helps)
Ugo MenaCommented:
I would most certainly attribute the DHCP issue on the clients to the Old, new, then Old DHCP server reboots. Did the route to the server change? Did you restart or flush the ARP tables from your switches?

I would guess that the clients had their IP lease from the old server, became confused when that server went offline, then tried to connect with the new one... could not, then just became isolated with a 169..... address (not being able to contact the DHCP server). Restarting the client forced them to look for a new DHCP server...

You may want to try restarting the router or switches at your remotes in order to flush any address tables they may have cached.

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