Turn on proxy_arp for this device. This is mainly useful for creating pseudo-bridges between a real interface and a virtual network such as VPN or a virtual machine for interfaces that don't support real bridging (most wlan interfaces). As ARP proxying acts slightly above the link-layer, below-ip traffic isn't bridged, so things like DHCP won't work. The advantage above using NAT lies in the fact that no IP addresses are shared, so all hosts are reachable/routeable.
WARNING: turns on ip-routing, so if you have multiple interfaces, you should think of the consequence and setup firewall rules to limit this.