Existing users, 
I launch and it shows a dialog with:
"Error 51: UNable to communicate with the VPN subsystem.
Please make sure that you have at least one network interface that is current active and has an IP address and start this application again."
(not "network interface" but "network configuration" btw)
and the OK quits the application... It started doing this seemingly for no reason after working the day before. Manually removed the software, reinstalled, no difference.
Created a separate Network configuration with nothing but the Airport active. No effect. Did this because Cisco fails when "Parallels Host-Guest" network configuration is active.
The system clearly has a working network configuration and associated IP address... I am entering this feedback...
Cisco VPN Client
IPsec VPN client (login required).
Version: 4.9.01.0180
Broken again, "Error 51"
Feedback Type: Commentary
Contributed by: Wm. Cerniuk Sunday, July 15 2007 @ 12:35 PM PDT
Product Platform: MacOSX
Used Product For: 1-6 months
Recommend Product: NO
Comments
Broken again, "Error 51" - ben.juang
I think one of the recent software updates broke it (AirPort Extreme Update 2007-004, or Security Update 2007-007 (10.4.10 Universal)?)Thursday, August 02 2007 @ 06:46 AM PDT
Broken again, "Error 51" - ollie2
try this solution, worked great for me!http://cb1inc.com/2007/06/11/fixing-cisco-vpn-client-4.9-with-parallels-desktop-3.0-on-mac-os-x
Tuesday, August 07 2007 @ 03:56 AM PDT
Broken again, "Error 51" - mark402
I don't run parallels, nor do I have any other of the configurations that are commonly being listed in the forums as possible causes for this error. But I still couldn't run Cisco VPN client (any version up to 4.9.01 (0080)) no matter what I tried. The infamous Error 51 kept popping up.The tip in this article fixed it:
http://cb1inc.com/2007/06/11/fixing-cisco-vpn-client-4.9-with-parallels-desktop-3.0-on-mac-os-x
Tuesday, September 18 2007 @ 03:06 AM PDT
Broken again, "Error 51" - ps.rivans
This comment comes having tried all the suggestions on this list.I actually rang Apple and having had the exact Error51 message and spending all night messing with ifconfig. This is the the solution it took 5 mins and and any IT dummy can do it.
Apparently Leopard OS X does break the Cisco Systemsubnet.
So guess what; Mac have a link which enables to down load the latest Cisco version. Simply down load then install and it all works brilliantly!!!
No need to login etc, etc...
As for Parallels, well I shouldn't have bought it as Leopard has boot camp built into it so you don't need Parallels and saves all the hassle with audio drivers not working when running things like Cakewalk in windows.
Boot camp solves all these issues too!
Here is the link which AppleMAC gave me today!!
www.macupdate.com/info.php/id/10317/cisco-vpn-client
Tuesday, October 30 2007 @ 08:34 AM PDT
Broken again, "Error 51" - bruceicohen
I am trying to run VPN client 4.9 on a new Mac with the Intel dual chiprunning OS 10.4.10
The built-in airport card was removed from my Mac by my employer who
replaced it with a removable USB 11g stick (wireless network adapter from
MacWireless.com). The USB wireless stick does work, and I have
a connection with the Internet. However, VPN client 4.9 gives the infamous
error 51 and makes no connection. An awkward remedy: With the wireless stick plugged in, if I reinstall the VPN client software, I can then connect with VPN, disconnect, reconnect, etc.. with no problems. I only have trouble if I remove the USB wireless adapter or restart the computer. At which point I have to reinstall
the VPN client to get it to work.
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Sunday, July 29 2007 @ 12:39 PM PDT