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Background and Review
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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