With IPHost Network Monitor you can run simple snmp requests against a Cisco device in your network.
c3gSubnetMask
Cisco 3G Subnet Mask
1.3.6.1.4.1.9.9.661.1.2.6.3.1.5
Subnet mask of the sector, not to be confused with the IP subnet mask.
Back to CISCO-WAN-3G-MIB MIB page.
IPHost Network monitor allows you to monitor c3gSubnetMask on Cisco device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Cisco firewalls right now.