Having trouble with using variables in functions and after functions..

Question

Morning,
I have simplified this, as it represents the core of what’s I obviously don't know how to do. 
Problem:
Say I populate that variable by reading in a list if IP addresses from file, finding the line that matches the hostname and place that string into the $IPAddress variable for use later throughout the script.   I did not run into a significant problem
until I decided to convert the routine of looking up the IP address and populating the variable into a function.   As soon as I made it a function, and called that function, the $IPAddress variable only had the data in it until the function completed. 
Once the function had done its work, and had placed the data into the variable, I expected that “new” data to be available after the function had completed..in that variable.  But it’s like the $IPAddress variable in the main script is a totally
different data then the variable with the same name that’s used within the function. 
So I think this demonstrates some of what’s kicking my butt.. 
$IPAddress = "This is the wrong data”
Function TestFunction () 
$IPAddress  #call to display the contents of the variable at the start of the function. 
$IPAddress = Read-Host -Prompt "Please enter the text ‘Goober’" 
$IPAddress   #call to display the contents of the variable at the end of the function. 
TestFunction
$IPAddress ​
The output I get is: 
This is the wrong data
"Please enter ‘Goober’"  : where I enter Goober
Goober
This is the wrong data
Huh? 
I had expected:
This is the wrong data
"Please enter ‘Goober’"  : where I enter Goober
Goober
Goober
What good is the repeatability of a function if it’s output is lost? 

Answer 1

THis seems to work.. IS there any reason NOT to do this?
$Global:IPAddress= "This is the wrong data” #"
Function TestFunction{
    Write-Host $IPAddress-fore green
      $host.ui.RawUI.FlushInputBuffer()
     $Global:IPAddress =
Read-Host -Prompt
"Please enter the text ‘Goober’"
    return$Global:IPAddress
TestFunction
return$Global:IPAddress