Detect external IP and track it in a text file. If the IP is changed during a periodic script run, send an email that your external IP has changed.

Check_External_IP.ps1

#
# PowerShell script that will check your external IP address and compare it to
# the one saved in c:\ip.txt. If a change is detected, use GMail to send you
# an email. 
#
# Author: Pat O'Brien
# Created: July 16, 2015
#

function sendMail($newIP) {
	$gmailUsername = "Your GMAIL Username"
	$gmailPassword = "Your GMAIL Password, yes in plaintext"
	$password = ConvertTo-SecureString $gmailPassword -AsPlainText -Force
	$credentials = New-Object -TypeName System.Management.Automation.PSCredential -argumentlist $gmailUsername, $password
	$From = "Who to send the email as"
	$To = "Who to send the email to"
	$Subject = "IP Address Change"
	$Body = "Your IP Address has changed. Your IP is now $newIP"
	$SMTPServer = "smtp.gmail.com"
	$SMTPPort = "587"
	Send-MailMessage -From $From -to $To -Subject $Subject -Body $Body -SmtpServer $SMTPServer -port $SMTPPort -UseSsl -Credential $credentials
}

# Find new IP
$newIP = (Invoke-WebRequest http://ipinfo.io/ip).content
$newIP = $newIP -replace "`t|`n|`r","" # Remove tabs, newlines and carriage returns from the content

# Load current IP from text file if it exists. If it doesn't exist, create it. 
if (Test-Path "c:\ip.txt") {
	$oldIP = Get-Content "c:\ip.txt"
} else {
	New-Item "c:\ip.txt" -type file
	"127.0.0.1" | Out-file c:\ip.txt
}

# Compare and send email if there is a change
if ($oldIP -ne $newIP) {
	"Saved IP of $oldIP doesn't match the new IP we found of $newIP. Sending email that the IP has changed."
	$newIP | Out-file c:\ip.txt
	sendMail($newIP)
}