I can confirm a few things.
At least in my case this is not a WiFi issue.
It isnt a DNS issue either.
In the case of my machine the ethernet connectivity to my ADSL router is compromised.
Renewing lease on my ethernet connection doesn't seem to make any difference, and is actually misleading - because I believe the lease isnt renewed at all - afterward I still cannot ping the router.
Netstat, or traceroute also do not work. I get weird memory assignment messages when i try to do this.
The fix for me is to remove the ethernet option in System Preferences>Network and immediately adding it in again.
Everything then works fine, without touching anything else on my iMac.
I believe this is a software issue - where the mac - upon wakeup, doesnt start up the ethernet port (and perhaps wifi) correctly.
I'm tempted to try a wired keyboard to eliminate the possibility that my Bluetooth keyboard is the underlying cause - just to see if this issue happens less frequently.
I'll update this thread with my findings - if anyone is interested.