CIDR_Resolver.py

from netaddr import *
import pprint
import socket

data = raw_input("Enter the CIDR you would like me to resolve: ")
network = IPNetwork(data)

file = open ("hostnames.txt", "a")

for ip in network:
    resolveip = str(ip)

    try:
        socket.inet_aton(resolveip)
    except Exception as invalid_ip:
        print "%s did not resolve" % invalid_ip
    else:
        hostname = socket.getfqdn(resolveip)
        if hostname == resolveip:
            pass
            print resolveip + " - No Hostname Found"
        else:
            file.write(hostname + "\n")
            print hostname

As discussed offline, here's a refactor for efficiency and migration to Python 3-
https://gist.github.com/clairmont32/5c5662ee7ff1bc799494fc43d8b5e3c2

Keep in mind if you're expanding a massive network, this could be even more efficient using threads with each thread assigned to process X number of hosts.