Hi,
can i use IP blocks 11.0.0.0/24 - 20.0.0.0/24 for our backbone and client units?
You can use whatever IP addresses you want, but you should not !!
On you private network you should use addressesses reserved for private use.
If you are not sure, consult google, wiki…
If you read the article that satman linked to, pay attention to the carrier grade NAT paragraph.
This is probably what you want to use.