Solution 1:

You can use the user network emulation, which is slow, but is simple and should be activated by default indeed.

qemu -net nic -net user ...

This is how it's done.

This solution have some drawbacks:

  1. Some protocole (ICMP) can't reach the internet, but tcp and udp can.
  2. From the host you can't reach the guest afaik
  3. Networking in the is slow compared to bridging