A Bridge is a software device that uses packet forwarding in a packet-switched network. Bridging allows multiple network interface devices to share the connectivity of one NIC and appear on a network as separate physical devices. The bridge examines a packet's source addresses to determine relevant target addresses. Once the target address is determined, the bridge adds the location to a table for future reference. This allows a host to redirect network traffic to virtual machine associated VNICs that are members of a bridge.
In Red Hat Virtualization a logical network is implemented using a bridge. It is the bridge rather than the physical interface on a host that receives an IP address. The IP address associated with the bridge is not required to be within the same subnet as the virtual machines that use the bridge for connectivity. If the bridge is assigned an IP address on the same subnet as the virtual machines that use it, the host is addressable within the logical network by virtual machines. As a rule it is not recommended to run network exposed services on a virtualization host. Guests are connected to a logical network by their VNICs, and the host is connected to remote elements of the logical network using its NIC. Each guest can have the IP address of its VNIC set independently, by DHCP or statically. Bridges can connect to objects outside the host, but such a connection is not mandatory.
Custom properties can be defined for both the bridge and the Ethernet connection. VDSM passes the network definition and custom properties to the setup network hook script.