Look like you are on Debian based distribution.
There is a tool called netmask:
sudo apt-get install netmask
Useful to figure the netmask when you are using CIDR or even Cisco netmask. It detect wrong netmask as well:
$ netmask -s 192.168.15.148/24
192.168.1.0/255.255.255.0
$ netmask -c 192.168.15.148/225.225.225.0
netmask: invalid mask 0xe1e1e100 from "225.225.225.0"
Quicker than reading a netmask table, it show the range of available IP in a network:
$ netmask -r 192.168.15.148/30
192.168.15.148-192.168.15.151 (4)