There are many reasons why you would do this, most of which are for testing and learning. Not going to go into why – just going to do it.

nmcli con add type dummy ifname dummy0 con-name dummy0 ipv4.address 10.101.101.1/24

nmcli con up dummy0

See the man pages for nmcli for options of setting up ipv6

if you want more – then change dummy0 to dummy1 or a higher number.

Set up a dummy bridge to route between virtual machines

nmcli con add type dummy ifname ifDummy1 con-name ifDummy1 ipv4.method disabled ipv6.method disabled connection.autoconnect-priority -999

nmcli con add type bridge ifname brDummy1 con-name brDummy1 ipv4.method manual ipv4.address 10.197.197.1/24 ipv6.method disabled

nmcli con mod ifDummy1 master brDummy1

Have fun!!