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linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

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</TABLE><P>The NIC also allows special letters to identify the country of the company or organization. Designators exist for all countries in the world, such as &quot;.ca&quot; for Canada and &quot;.uk&quot; for the United Kingdom.



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<P>Not all companies that are outside the U.S. h<A NAME="I2"></A>ave country identifiers. To some extent, the date of registration may affect the use of the country identifier, as companies that joined the Internet when it was still relatively uncrowded would have been given a standard identifier. Also, some non-U.S. corporations use a U.S.-based company to register for them, giving them a choice of using a country designator or not.



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<P>The Domain Name System (DNS) is a service provided by the TCP/IP family of protocols that helps in the addressing of messages. When you address mail to bozo@clowns_r_us.com, the DNS system translates this symbolic name into an IP address by looking up the domain name in a database. DNS lets you forget about those IP addresses, allowing much simpler names: the domain name. The usual syntax for sending a message to a user on the Internet is username@domain_name, as the &quot;bozo&quot; example shows. (DNS doesn't have to run on top of TCP/IP, but it usually will on Linux systems.)



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<P>If a company decides not to get their own domain name but will use an on-line service (such as CompuServe or America Online), a unique domain name is not needed. Instead, the domain name of the service provider is part of the address. A user is then identified by a name or number of the service provider, such as [e-mail].



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<P>In practice, when you send a symbolic name to DNS, it doesn't check the user's actual host, otherwise there would be millions of IP addresses in the database. Instead, DNS is concerned with only the network part of the address, which it translates to the network IP address and sends out over the network. When the receiving network's Internet machine receives the message, it uses an internal database of its own to look up the user's host and takes care of that part of the trip.



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<FONT SIZE=5 COLOR="#FF0000"><B>Network Basics</B></FONT></CENTER></H3>



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<P>A network topology is the way the cabling is laid out. This doesn't mean the physical layout (how it loops through walls and floors) but rather how the logical layout looks when viewed in a simplified diagram. You may hear many different names for the type of network you have: ring, bus, star, and so on. They all refer to the shape of the network schematic.



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<FONT SIZE=4 COLOR="#FF0000"><B>Network Topologies</B></FONT></CENTER></H4>



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<P>One of the most widely used network topologies (and the one most often used in medium to large Local Area Networks) is the bus network. A bus network uses a cable to which are attached all the network devices, either directly or through a junction box of some sort. Each device uses a transceiver to connect to the backbone. The manner of attachment depends on the type of bus network, the protocol in use, and the speed of the network. The main cable that is used to connect all the devices is called the backbone. Each end of the backbone (also called a bus) is terminated with a block of resistors or similar electrical device. Most large TCP/IP networks used by Linux are bus topologies.



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<P>A popular variation of the bus network topology is found in many small LANs. This consists of a length of cable that snakes from machine to machine. Unlike with the bus network, there are no transceivers along the network. Instead, each device is connected into the bus directly using a T-shaped connector on the network interface card, often using a connector called a BNC. The connector connects the machine to the two neighbors through two cables, one to each neighbor. At the ends of the network, a simple resistor is added to one side of the T-connector to terminate the network electrically.



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<P>This machine-to-machine network is not capable of sustaining the higher speeds of the backbone-based bus network, primarily because of the medium of the network cable. A backbone network can use fiber optics, for example, with small coaxial or twisted pair cables from a junction box to the device. A machine-to-machine network is usually built using twisted pair or coaxial cable. (With Ethernet, these cables are called 10Base-T and 10Base-2 respectively.) Until recently, these networks were limited to a throughput of about 10 megabits per second (Mbps). A recent development of two slightly different protocols called 100VG AnyLAN and Fast Ethernet allows 100mbps on this type of network, though. Most small Linux networks use this topology, because it is inexpensive, easy to wire and control, and relatively simple to administer.



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<P>The problem with the type of bus network that involves T-shaped connectors between the backbone cable and the PC is that if one connector is taken off the network cable or the network interface card malfunctions, the backbone is broken and must be tied together again with a jumper of some sort. This can cause erratic behavior of the network or a complete failure of all network traffic. To help avoid this problem, an alternate method of connection to the network can be used. This method employs boxes with a number of connectors (usually similar to telephone jacks, but wider) that connect between the box and the PC network card. Disconnecting a cord from a PC to the box doesn't compromise the integrity of the network as the box is still handling the backbone properly. The 10Base-T Ethernet system uses this approach.



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<P>Another network topology is the ring network. Although most people think the network cable is made into a physical loop joining into a large circle, that's not the case in the most common form of ring network called Token Ring. The ring name comes from the design of the central network device, which has a loop inside it to which are attached juntions for cables of all the devices on the network.



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<P>With a Token Ring network, the central control unit is called a Media Access Unit, or MAU. Inside the MAU is the cable ring to which all devices are attached, and which is similar to the backbone in a bus network. IBM's Token Ring is the most commonly encountered network system that uses a ring topology. Linux networks can be set up to use Token Ring, but few are.



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<P>A star network is arranged in a structure that looks like a central star with branches radiating from it. As you will see shortly, this is a common layout with twisted-pair peer-to-peer networks. The central point of the star structure is called a concentrator, into which plug all the cables from individual machines. One machine on the network usually acts as the central controller or network server. A star network has one major advantage over the bus and ring networks: when a machine is disconnected from the concentrator, the rest of the network continues functioning unaffected. True star networks are very seldom used for Linux, although a single Linux server in the center of a number of terminals or PC clients is, in a sense, a star network. In some networks, such as 10Base-T (twisted pair), that use a box with cables snaking off to each device, the box and cables make up a small star network, albeit in a larger bus topology.



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<P>The last type of topology that concerns us is called the hub network. It is similar to the bus network in that it uses a backbone cable that has a set of connectors on it. The cable is called a backplane in a hub network. Each connector leads to a hub device, which leads off to network devices. This allows a very high speed backplane to be used, which can be as long and complex as needed. Hub networks are commonly found in large organizations that must support many network devices and need high speeds.



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<P>The hubs that lead off the backplane can support many devices, depending on the type of connector. They can, for example, support hundreds of PC or Macintosh machines each, so a hub network can be used for very large (tens of thousands of network devices) networks. The cost of a hub network is high, though.



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<FONT SIZE=4 COLOR="#FF0000"><B>Network Media</B></FONT></CENTER></H4>



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<P>The type of cabling used in a network is called the network medium. Today, networks use many types of cables, although only a few types are in common usage. Some of the more exotic types of cables are very expensive. The type of cabling can have an influence on the speed of the network, although for most small- to medium-sized local area networks this is not a major issue.



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<P>Twisted pair cabling is one of the most commonly used network mediums because it is cheap and easy to work with. Unshielded twisted pair cables&#151;often called UTP&#151;look just like the cable that attaches your household telephone to the wall jack. Twisted pair cables have, as its name suggests, a pair of wires twisted around each other to reduce interference. There can be two, four, or even more sets of twisted pairs in a network cable.



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<P>Twisted pair cables usually attach to network devices with a jack that looks like a telephone modular jack but is a little wider (supporting up to eight wires). The most commonly used jacks are called RJ-11 and RJ-45, depending on the size of the connector (and the number of wires inside). The RJ-11 connector is the same as the modular jack on household telephones, holding four wires. The RJ-45 jack is wider than an RJ-11 and holds eight wires.



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<P>A variation on unshielded twisted pair cables is shielded twisted pair, often called STP. The shielded twisted pair cable has the same basic construction as its unshielded cousin, but the entire cable is wrapped with a layer of insulation for protection from interference, much like a cable for connecting speakers to your stereo system. The same types of connectors are used with both forms of twisted pair cables.



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<P>Twisted pair cables have one major limitation. They only support one channel of data. This is called baseband or single channel cabling. Other types of cables can support many channels of data, although sometimes only one channel is used. This is called broadband or multiple channel cabling. You can use twisted pair cabling for a small Linux network without any problem, but a much easier and more economical (in the long run) system is to use coaxial cable.



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<P>Coax cable is designed with two conductors, one (usually a number of strands intertwined) in the center surrounded by a layer of inner insulation, and the second a mesh or foil conductor surrounding the insulation. Outside the mesh is a layer of outer insulation. Because of its reduced electrical impedance, coaxial is capable of faster transmissions than twisted pair cables. Coax is also broadband, supporting several network channels on the same cable.



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<P>Coaxial cables come in two varieties: thick and thin. Thick coax is a heavy, usually yellow, cable that is used as a network backbone for bus networks. This cable is formally known as Ethernet PVC coax, but is usually called 10Base5. Because thick coax is so heavy and stiff, it is difficult to work with and is quite expensive. Linux systems that are attaching to a large commercial network may have to use thick coax, but this is very seldom encountered.



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<P>Thin coax is the most common type used in Ethernet networks used by Linux. It goes by several names, including thin Ethernet, thinnet, 10Base2, and somewhat derogatorily as cheapernet. Formally, thin coax is called RG-58. Thin coax is the same as your television cable. The inner connector can be made of a single, solid copper wire or fashioned out of thin strands of wire braided together. Thin coax is also used in other networks like ARCnet, although the specification is known as RG-62.



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<P>Thin coax is quite flexible and has a low impedance so it is capable of fast throughput rates. It is not difficult to lay out, because it is quite flexible, and it is easy to construct cables with the proper connectors at each end. Thin coax is broadband, although most networks use only a single channel.



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<FONT SIZE=4 COLOR="#FF0000"><B> Networking Hardware</B></FONT></CENTER></H4>



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<P>Establishing a Linux TCP/IP network requires two components of hardware: the network interface card (NIC) and the network medium. As with most things in life, you can opt to go for an inexpensive card and medium or spend lots more money for full-featured items. As the amount you spend increases, you get more functionality and benefits, although usually with diminishing returns for your dollar.



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<P>Before deciding on the type of network interface cards to use (unless you already have some), you should decide on the type of cabling the network will use so the connectors on the network interface card can match. 10Base2 and 10Base-T are really the only two cables that you'll want to consider. The alternatives are much more expensive or require special hardware.



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<P>The most common choice is 10Base2 (Thin Ethernet). A 10Base2 network interface card has a male plug jutting out from the back to which is attached a T-shaped connector. Both ends of the T are attached to coaxial cables running to the two neighboring machines. If the machine happens to be at the end of the chain, a terminating resistor plug is attached instead (to electrically terminate the cable).



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<P>Once you've decided on the type of cabling, you can choose network interface cards with the proper connectors. (You can use RJ-45 connectors with a 10Base2 network, and vice versa, although you have to purchase special adapters which cost more than a new network interface card.) Don't forget to check the bus architecture (local machine, not network architecture) the card is designed for, because the same manufacturer may have ISA, EISA, and MCA versions in the same packaging. It is a good idea to check the hardware compatibility lists provided with each release of Linux to make sure that the network cards you are considering purchasing have been tested and found to work properly with Linux.



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<P>Many network interface cards provide more than one connector on the back plane of the card. It is not unusual to have both a 10Base2 BNC and 10Base5 AUI (thick Ethernet, used for much larger networks) connector on a single card, with either available for use. The same applies to RJ-45 and 10Base5 AUI connectors. A few cards provide 10Base2 BNC and 10Base-T RJ-45 connectors in addition to the 10Base5 AUI connector. These provide the ultimate in flexibility for the user and allow you to change your network from one format to the other as your network grows or shrinks.



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<P>You can find network interface cards on the market from many different manufacturers, including brand name vendors such as Novell, Artisoft, Intel, SMC, and Hewlett-Packard; original equipment manufacturers (OEM); and some lesser-known brands, which are usually cheaper. Most network interface cards are clearly labeled with the type of connector (RJ-45 or BNC) and the network systems it supports.



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<P>Some network interface cards require you to manually select the card's parameters, such as interrupt (IRQ) and interface address, by changing jumpers on the card. This can be daunting for novices, as well as difficult if you are not sure whether a setting will conflict with another card in your system. Watch the boot messages from Linux to see what the potential conflicts might be, use a DOS-based utility, or simply experiment! It doesn't hurt Linux to change settings, although you may end up with a frozen machine that requires rebooting and cleaning.



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<P>More recently, auto-configuring cards have been appearing (such as the Intel EtherExpress series) which use DOS- or Windows-based software to set the parameters. You can use these features on a Linux system by booting into DOS (from a DOS boot disk or a DOS partition) to set the parameters, and then moving back to Linux and use the set parameters. Linux will recognize the card at those settings, as the hardware responds to its requests.



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<FONT SIZE=5 COLOR="#FF0000"><B>Summary</B></FONT></CENTER></H3>



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<P>This chapter has covered a lot of ground, ranging from TCP/IP to network cabling. However, now that the basics are out of the way, the next few chapters can focus on networking with Linux and connecting to the Internet.



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