Improvement on https://gist.github.com/Azlirn/3fc0e72cbb85d24e2b6139791e59784b for efficiency and migrating away from Python 2.X

dnsResolver.py

import re
import socket
import ipaddress

# gather range from user
cidrNetwork = str(input('Enter a CIDR range to resolve:\n'))

# exit if input range does not match a (basic) valid IP/notation format
while True:
    if not re.fullmatch('^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}/\d{1,}', cidrNetwork):
        exit('Enter a valid CIDR range.\n')
    break

# explode the network into hosts
try:
    hosts = [ip for ip in ipaddress.ip_network(cidrNetwork).hosts()]
except Exception as err:
    print(err)
    exit()


# do a basic resolution, append non-initial values
resolved = []
for x in hosts:
    x = str(x)
    reverse = socket.getfqdn(x)
    if reverse is not x:
        resolved.append(reverse + '\n')


print('Writing results to file...')

# do a basic write and error check. This could be more exception safe but will do
try:
    with open('resolved_hosts.txt', 'wt') as file:
        file.write(''.join(resolved))

except Exception as Err:
    print(Err)

print('Done')