We will use a server with a single
Ethernet card running our base Fedora Core 6 system. The default networking on
this box is shown in the following diagram. As we go through the various options
in this chapter, we will modify this diagram to indicate the changes made to the
network interfaces and configuration while using Xen.
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Networking
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Bridged Networking
Bridging is a technique used for connecting different network segments. It is the
default option for Xen networking as it simply connects all the virtual machines
to the outside world through virtual network interfaces connected to the bridges
created by Xen. Bridges connect two LANs (local area networks) together and
forward frames using their MAC (media access control) address. They thus operate
at the lowest level of the network layer and are completely unaware of something
like an IP. The following diagram depicts a simple network bridge.
A network bridge is primarily utilized to connect multiple network segments by
using the MAC addresses for addressing. It uses broadcasting to locate devices that
are not known to it. Once the devices have been located, their MAC addresses are
stored in a bridge table, which is the first place checked when a new packet frame
comes in.
The most common form of bridging used on Ethernet networks is transparent
bridging. The systems using the bridge are not aware of its existence as the bridge
transparently passes traffic from one network segment to another based on
specific MAC addresses.
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