传统的以太网是一个广播型网络，网络中的所有主机通过HUB 或交换机相连，处在 同一个广播域中。HUB 是物理层设备，没有交换功能，接收的报文会向所有端口转 发；交换机是链路层设备，具备根据报文的目的MAC 地址进行转发的能力，但在 收到广播报文或未知单播报文（报文的目的MAC 地址不在交换机MAC 地址表中） 时，也会向除交换机报文入端口之外的所有端口转发。 上述情况使网络中的主机会收到大量并非以自身为目的地的报文，在浪费大量带宽 资源的同时，也造成了严重的安全隐患。 隔离广播域的传统方法是使用路由器，但是路由器成本较高，而且端口较少，无法 划分细致的网络。 为解决以太网交换机在LAN 中无法限制广播的问题，出现了VLAN（Virtual Local Area Network，虚拟局域网）技术。-err

All the code examples in this book are written in C#, Microsoft s flagship new programming language developed concurrently with the .NET Framework. Our hope is that Visual Basic developers won t feel alienated by this focus on C#. In fact, C# and Visual Basic code are structurally very similar and, apart from the obvious language syntax differences, the C# samples should be very readable to a Visual Basic .NET developer. On the companion CD, you ll find every sample application in this book with versions in C# and Visual Basic .NET. The only exceptions to this are the custom control examples from the second half of Chapter 15 and from Chapter 16. These are only in C#, not because you can t use Visual Basic (or any other language the .NET Framework supports, but because we didn t have time to do it!

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