I have set up the openstack couple times now. But, I still have some problem with the networking bits in Openstack. Today, I found the wiki page from openstack.org which clearly explained what that is.
Basically, the machine that has the nova-network will take care of all the network related stuff in openstack. For example, It will:
1. Give out the IP address to the VMs and keep track of it.
2. The br100 interfaces is used to bridge the private network (–fixed_range=x.x.x.x) that we define in the nova.conf file to the public. This interface can also be set up on the eth0 interface (if you only have one network interface.
3. Nova-Network will use that to route VM’s network traffic to the public .
The second article I found discussed couple options about HA for nova-network to prevent its single point of failure which is very interesting as well.
 Understanding FlatNetworking:
 Networking in Nova