Post by David ChristensenThank you for your replies.
The underlying issue appears to be that my old-school Linux console
network administration skills have been rendered obsolete by systemd
and
Post by David ChristensenNetworkManager.  I typically install Xfce when installing Debian,
and
Post by David Christensenthe panel NetworkManager Applet has made it too easy to "monkey see,
monkey do" without understanding the details.
What I really need is a good book
Thanks for that question about books.
A 'book' ?  What is a 'book' ??
I recall mother reading from such things when I was a child.
(so hard to keep a straight face and not be laughing as I try to
pretend that 'books' are old school, but sadly it is not far from the
truth. My children know not the age before 'the Internet and Google
were how to find knowledge', the idea of going to a library to
research though 'books' seem so foreign to them. Soon asking an AI
system for the answer will be the way, and Google and Internet
searching, will be thought of as historic research techniques.)
Post by David Christensenor document that explains the design
and implementation of networking with systemd and Network Manager on
modern Debian GNU/Linux systems.  Recommendations?
Sadly I have not found any documentation (or books) for any thing
Linux. It seems to be a 'piecing together of random statements' from
comments and howtos on the Internet, and much personal
experimentation.
The closest to any systematic learning method I have seen would be Red
Hat certifications. Not that I have even done any.
As far as general networking books, CISCO certification books have
been useful to me, but I am not a good reader, and they are lengthy.
For Internet doco, see pages like Â
https://wiki.debian.org/NetworkConfiguration#Setting_up_an_Ethernet_Interface
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_and_managing_virtualization/configuring-virtual-machine-network-connections_configuring-and-managing-virtualization
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_and_managing_networking/configuring-a-network-bridge_configuring-and-managing-networking#configuring-a-network-bridge-using-nmcli-commands_configuring-a-network-bridge
https://www.ibm.com/docs/en/linux-on-systems?topic=choices-kvm-default-nat-based-networking
https://wiki.libvirt.org/page/Libvirtd_and_dnsmasq
https://wiki.libvirt.org/page/Networking
If it helps, two things I have done is;
1) Set manual IP address in Network Manager so that it is managing the
networking on my behalf. Particularly if I use a GUI, of which my
favourite for VMs is XFCE as it is lightweight and works well with
spice (that is not a Dune reference, btw). Network Manager can even do
bridges, though I have had issues on initial power on.
2) I have had success, uninstalling Network Manager and then
configuring /etc/network/interfaces.
Below are two examples from my own 'play' test environments. Â
Over time I have used bridging many times, so my VMs can be on the
same network as my other PCs, servers, etc. Bridges are not necessary
when all VMs are test VMs in their own private network running in the
same Hypervisor. Some times I have  set up a route from my PCs' own
network to the Hypervisor's provided network for its VMs.
/etc/hosts             Â
/etc/resolv.conf      (Generated by NetworkManager, if
installed)
/etc/network/interfaces
Examples for Static IP addresses.
# cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
source /etc/network/interfaces.d/*
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
allow-hotplug enp1s0
iface enp1s0 inet static
    address 192.168.100.101/24
    gateway 192.168.100.1
    # dns-* options are implemented by the resolvconf package, if
installed
    dns-nameservers 192.168.100.1
    dns-search tstenv.net
# cat /etc/resolv.conf
search tstenv.net
nameserver 192.168.100.1
$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
source /etc/network/interfaces.d/*
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
#allow-hotplug enp1s0
auto enp1s0
iface enp1s0 inet manual
iface enp1s0 inet6 manual
# bridge
auto brRed
iface brRed inet static
    address 192.168.100.103/24
    gateway 192.168.100.1
        network 192.168.100.0
        broadcast 192.168.100.255
        bridge_ports enp1s0
        bridge_stp off
        bridge_fd 0
        bridge_maxwait 0
    # dns-* options are implemented by the resolvconf package, if
installed
    dns-nameservers 192.168.100.1
    dns-search tstenv.net
# bridge allow auto conf for ipv6
iface brRed inet6 auto
        accept_ra 1