Checks the external IP address of your router and emails you if it changes
This is useful if you access your dynamic IP network from outside of your (generally home) network.
I have tested it on a ReadyNAS, Windows 7, Ubuntu 16 with a DD-WRT router and gmail.
It will create a file the first time it runs that holds the current IP address.
Whenever it runs again (instructons for setting up a chron is in the script) it checks the extern IP address and compares it to the old one.
If it has changed, it will email you and update the current IP file.

The configuration file has two sections:
[Email]
FROM:yourEmailAddress@goes.here
TO:whereYouWantToSendAlerts@goes.here
PASSWORD:passwordWithoutQuotes

[Message]
SUBJECT:IP Address at home changed!  You can put %OLD_IP_ADDRESS% here too!
BODY:IP Address at home changed from %OLD_IP_ADDRESS%. Time to update your favorite ISP with
 %NEW_IP_ADDRESS%.  Note the preceeding space to designate a new line

Copy default.ini to another file and make the appropriate changes.

Feel free to change the subject and body.  New lines are created by adding a new line with a leading space.
Note that the subject and body may contain two keywords (%OLD_IP_ADDRESS% and %NEW_IP_ADDRESS%).
These keywords are replaced with the previous and current ones before sending the email (if the external IP address has changed).

Note that this script will NOT email you if there are no changes.

Run like this:
python CheckIp.py your_config_file.ini

To test, remove the "old_ip.txt" file and run.

There is a lot of room for testing as I have only tried it on a handfull of OS and only my DD-WRT router.  It should
work on other routers, but you know how it goes...

Feel free to create additional features!