app.py

from flask import Flask

app = Flask(__name__)

app.config['SERVER_NAME'] = 'flasksubs.local:5000'
app.config['SUB_DOMAINS'] = {
    'good': 'good sub domain',
    'ok': 'ok sub domain'
}
app.config['NO_SUB_DOMAIN_MSG'] = 'no sub domain found'


@app.route("/", defaults={'subdomain': ''})
@app.route("/", subdomain='<subdomain>')
def subdomain_message(subdomain):
    if not subdomain:
        msg = 'This is the root url'
    else:
       msg = app.config['SUB_DOMAINS'].get(subdomain)

    if msg is None:
        msg = app.config['NO_SUB_DOMAIN_MSG']

    return msg


if __name__ == '__main__':
    app.run(port=5000, debug=True)

hosts

127.0.0.1   flasksubs.local
127.0.0.1   good.flasksubs.local 
127.0.0.1   ok.flasksubs.local
127.0.0.1   bad.flasksubs.local