regarding your questions on diag sniffer:
10.99.19.12 -> 10.15.12.1: icmp: echo request
This means that IP 10.99.19.12 sent an ICMP packet to 10.15.12.1; echo request clarifies that this is a ping query (the echo response in the next line is the ping reply)
10.15.12.83.40820 -> 192.168.40.53: udp 29
this means that IP 10.15.12.83 with source port 40820 contacted IP 192.168.40 on port 53 using protocol UDP, and the packet contained 29 bytes. UDP port 53 is DNS traffic usually.
10.15.12.83.52751 -> 10.15.12.109.9100: syn 638097597
This means that IP 10.15.12.83 with source port 52751 contacted IP 10.15.12.109 on destination port 9100; this is a TCP SYN packet, the first packet in establishing a TCP connection; 638097597 is the sequence number (to keep track of what packets belong to what TCP session). TCP 9100 is not a standard port so we can't say what kind of communication beyond basic TCP is happening, but to my knowledge several printer companies (like HP) use that port for various purposes
I hope this helps!
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