There are two execution modes in QEMU: non-root (default) and root requires sudo or root permission. The difference in the modes relates to virtual network configuration.
In non-root mode QEMU sets up an internal virtual network which restricts network traffic passing from the host and the guest. This works similar to a NAT router. You can not access this network unless you redirect tcp ports.
In root mode QEMU creates a subnet on a virtual Ethernet adapter, and relies on a DHCP server on the host system.
The following sections detail how to use the modes, including redirecting the non-root mode so it is accessible from your local host.