rubbish-python3-app.py

#!/usr/bin/env python
#
# Requirements: 
#   Flask==0.11.1

import ipaddress

from flask import Flask, request
app = Flask(__name__)

@app.route('/')
def entry():

    ip_network = ipaddress.ip_network('172.16.0.0/12')
    ip_address = ipaddress.ip_address(request.remote_addr)

    if ip_address in ip_network:
        return "Welcome {}".format(ip_address)
    else:
        return "Forbidden {} not in Network {}".format(ip_address, ip_network)