[Neutron][segments]

Hello, I am reading about openstack neutron routed provider networks. If I understood well, using segments I can create more subnets within the same provider net vlan id. Correct? In documentation seems I must use different physical network for each segment. I am using only one physical network ...in other words I created an openvswitch with a bridge (br-ex) with an interface on which I receive all my vlans (trunk). Why must I use different physical net? Sorry, but my net skill is poor. Ignazio