h a switch separates the two networks and both of them have all the transmission bandwidth available. When more than one switch exists in an Ethernet network, the spanning tree algorithm is used for frame forwarding through the switches network up to the correct destination. The spanning tree algorithm, besides CSMA, is a base characteristic of Ethernet net- working and many discussions on the scalability of the Ethernet to the public network are centered on the potentialities and weakness of the spanning tree protocol. For this reason, this protocol is described with some details in Appendix B. At a first look, it could seem that a switch is somehow similar to a router, but it is not true, a part for the fact that both machines implement routing at their own layer. A router deals with IP datagrams: at the router input, an IP envelope is open, the IP address read and, in case of IPv4, the checksum calculated. Moreover, in public networks where real-time transmission protocols are used, the router manages QoS by controlling packets delays and transmission quality. Last, but not least, since it terminates the Internet link layer, a router can be used in the home of a customer to separate the customer private home network from the public net- work, SO separating what is the responsibility of the carrier from what is the responsibility of the customer. If the customer has a private LAN, the router implements the NAT procedure to allow machines in the private LAN to enter the external Internet and it is able to connect with the remote authentication machine (the broadband remote access server [BRAS], that is another router, see Chapter 7) to authenticate the user when entering into the public Internet and, in case, to perform billing and other user account related functions. A switch is a much simpler machine that is not able to perform all the operations we have listed for a router: the only function of a LAN switch is to separate, from the point Networks Fundamentals and Present Architectures of view of the transmis