This problem can occur two way, one is cause by the server not updateing your gateway routers arp tables. You can solve this problem by logging into your router and clearing the arp tables. for instace, on a cisco router your would do the following:
using serial or hyperterminal if available
- connect to your router
- password: (enter password)
- router> enable
- password: (enter password)
- router# clear arp
- router# write mem
- router# exit
done.
The other problem is caused when using Network Load Balancing (NLB) on windows 2008 / 2008 R2 Server
For more information on this issue use these links:
http://support.microsoft.com/kb/960916
http://blogs.technet.com/b/networking/archive/2009/01/15/unable-to-connect-to-windows-server-2008-nlb-virtual-ip-address-from-hosts-in-different-subnets-when-nlb-is-in-multicast-mode.aspx