Comment:  I have been using PuTTY on Windows 10 to communicate with the Raspberrypi V4.  I was confused about using PUTTY in some cases.  Here are the things I WISH I knew before starting to use PuTTY:
———————————( About PuTTY)——————-
              When you installed the operating system with the Raspberrypi Imager you picked a USERNAME and PASSWORD which you will use to log into your Pi using SSH on PuTTY.. Remember those…  NOTE: When you type in your password to log in, NOTHING will show at the prompt.  Type the password carefully and hit Enter.

• Install PuTTY as shown on page 39 of the book.

NOTE: The Windows PuTTY install includes PuTTY and some other utilities and the PuTTY Manual.
NOTE: It is possible to open more than one PuTTY SSH session on your Raspberrypi at the same time. You will do that to test your MQQT install.  After the first session (#1) is running,  click/start PuTTY again. It will open a second window (#2). You need to log in with USER and PASSWORD same as first session.  You can resize and arrange the two windows to test MQQT.
NOTE: You can cut and paste things to / from PuTTY but it works very differently than windows:
PuTTY COPY:  Click and drag over the part of the PuTTY screen you want to copy, then release the mouse button.  The area you highlighted is ON THE CLIPBOARD.  Do NOT use CTRL-C or CTRL-V ! Go to some other windows app like Word, and then use CTRL-V to paste there.
PuTTY PASTE:  Go to the source you want to copy (Usually the RNT book).  Use CTRL-C to copy a command you want to use on PuTTY. On the PuTTY window at the prompt, press  Right Mouse Button.  That is Paste on PuTTY. Who Knew  ??  Makes aged Typo-Prone users Happy!
—————————-(  end PuTTY)——————————————-
A Few Funny Confusions:
PG103—– MQQT Test:
When running “mosquito –v” there are many lines of response. Including:
“Listen IP04 on port 1883 
Error: Address already in use “
“Listen IP06 on port 1883 
Address already in use”
 
? Is that any problem?
 
PG104—– MQQT Test setup: 
After  starting session#1 to subscribe with “mosquitto_sub -d -t testTopic”The screen shows an unending series of:

• PINGREQ
• PINGRESP

Later when a message is published from session #2 the result is displayed OK, then pings continue.   
MQQT Works!
?Just did not  expect that Ping activity ?
 
PG105 The next activity will be: “Enable remote access /authorization” 
— But there are two open PuTTY windows and one is running Pings..And user needs to enter commands, use editor etc.
?? Best to just close those two windows at this point and open a new session??
OK ON TO PAGE 105….
 
THANKS for your patience…   Terry