Network BasicsNote - if you specify any networking options on the command line (via -net or -netdev) then QEMU will require you to provide options sufficient to define and connect up both parts.
(Forgetting to specify the backend or the network device will give a warning message such as "Warning: netdev mynet0 has no peer", "Warning: vlan 0 is not connected to host network" or "Warning: vlan 0 with no nics";
the VM will then boot but will not have functioning networking.)

Note - if you are using the (default) SLiRP user networking, then ping (ICMP) will not work, though TCP and UDP will.
Don't try to use ping to test your QEMU network configuration!