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UNCLASSIFIED Background and Review Version 1.0g  UNCLASSIFIED 15   2.  Background and Review This section reviews some background information about TCP/IP networking, router hardware architecture, router software architecture, and network security. In order to keep this section brief, it glosses over a lot of issues. To compensate for that briefness, the reference list at the end of the section includes a long list of other useful sources of background information. Readers with a good grasp of network and router fundamentals may want to skip this section, but since it is relatively brief, why not humor the author and read on.   2.1.  Review of TCP/IP Networking As mentioned in Section 1.1, on a small computer network, it is feasible to use simple broadcast or sequential (token) mechanisms for moving data from point to point.  A local area network is composed of a relatively small number of hosts connected over a relatively small physical area. “Relatively small” is the important phrase here. To give some meaning to the term “relatively,” consider that a 10BaseT Ethernet (10 megabit per second using twisted pair cabling) has a usual maximum of 1024 stations over a maximum cable distance of 2500 meters. For instance a typical office LAN, using 100BaseT Ethernet, might have 100 computers (and printers) attached to a switch or set of hubs. An Ethernet local area network (LAN) is essentially a (logical) bus based broadcast network; though the physical implementation may use hubs (with a physical star topology).  As one would expect, broadcast LANs must deal with collisions; either by preventing them or detecting them and taking appropriate action. Token based LANs avoid collisions by only allowing one host at time to transmit (the host that currently has the token may transmit). Standards that relate to LANs are primarily the IEEE 802.x series. For instance, 802.3 is the Media Access Control (MAC) standard for CSMA/CD (the Ethernet standard); while 802.5 is the MAC standard for Token Ring. Just above the MAC level is the Logical Link Control (802.2) standard and above that it the High Level Interface (802.1) standard.   Within a LAN, addressing is done with a MAC address. Between LANs using TCP/IP addressing is done using IP addresses. If you are lost at this point, keep reading because much of this will be explained below. If you are still lost at the end of Section 2, then consider reading parts of some of the books and/or web pages listed at the end of the section. 2.1.1.    Purpose of a Router In larger, more complex computer networks, data must be directed more carefully.  In almost all cases, large networks are actually composed of a collection of LANs that are interconnected or “internetworked”.  This is where routers come in. Routers take