Assign all ports (WAN+LAN) to one VLAN?

Hi,

I have a local LAN and two wireless LANs (one is for guests).

Can I assign one VLAN to all WAN and LAN ports (private) and create one for guest WLANs? I would like to use the WAN port as the upstream port to the firewall.

Routing, DHCP etc is handled by the firewall.

Thanks for your help on this.