Installing ROS2 Humble (see the Details) on both Host (Raspberry PI 4 with Ubuntu 22.04 OS arm64) and Client (Intel I9 with Ubuntu 22.04 OS amd64) was easy, but the problems arose during testing.

Test using "talker" and "listener" as described here  ROS2 Humble - Ubuntu Install Debs went fine on the same machine but failed on different machines (e.g. talker on Client and listener on Host or viceversa). 

After some struggling (1 full day of "change" and "test") I realized that Multicast was enabled on both the machines on all their interfaces but it didn't work. 

The confirmation that Multicast didn't work on my network came from: 

ros2 multicast receive

 on a machine and

ros2 multicast send

 on the other one as "receive". In fact the receiver received ... nothing!

Reading a lot on the Web I realized that many WiFi routers need a setting, whose name not always is Multicast (it would be too easy) but uPnP. 

Fortunately my router had this setting and, after enabling it, all the tests worked fine.