In general the short answer is this is not practically feasible.
In the very specific case you have access to your firewall your firewall has a public IP address on its outside interface and you can access a shall prompt on the firewall. You can do something like this:
ssh firstname.lastname@example.org 'eval $(ipcalc -np `ifconfig eth1 | sed -n "s/inet addr:\([^ ]*\).*Mask:\([^ ]*\).*/\1 \2/p"`) ; echo $NETWORK/$PREFIX'
This command is specific to Redhat6.
Remember this information is available from the firewall because a network administrator entered the information provided him by the ISP to configure the outside interface. Therefore the only reliable way to get this information is to ask your ISP. Even if you can deduce the CIDR for your ISP from DNS and whois records, you can't deduce how the ISP has subdivided the available IPs internally. This is an administrative function and must be solved administratively.