lsg28.htm

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

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<P>The following protocols don't fall into the categories mentioned previously, but they provide important services over a network:



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<LI><B>NFS (Network File System)</B>  Allows directories on one machine to be mounted on another and accessed by a user as though they were on the local machine.



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<LI><B>NIS (Network Information Service)</B>  Maintains user accounts across networks, simplifying logins and password maintenance.



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<LI><B>RPC (Remote Procedure Call)</B>  Enables remote applications to communicate with each other in a simple, efficient manner.



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<LI><B>SMTP (Simple Mail Transfer Protocol)</B>  A dedicated protocol that transfers electronic mail between machines.



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<LI><B>SNMP (Simple Network Management Protocol)</B>  An administrator's service that sends status messages about the network and devices attached to it.



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<P>The different protocols within TCP/IP are maintained by a governing standards body that is part of the Internet organization. Changes to the protocols occur when new features or better methods of performing older ones are developed, but this is a rare occurrence and usually results in backward compatibility.



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<FONT SIZE=4 COLOR="#FF0000"><B>TCP/IP, the Internet, and Layered Architecture</B></FONT></CENTER></H4>



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<P>The Internet is not a single network but rather a collection of many networks communicating through TCP/IP. TCP/IP and the Internet are so closely interwoven that TCP/IP's architecture is often called the Internet architecture. Almost from the start of the Internet as the ARPAnet, it became obvious that existing protocols couldn't handle the sheer volume of traffic that the network had to carry, so a project was undertaken to develop new communications protocols.



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<P>The TCP/IP protocols were first proposed in 1973 and led to a standardized version in 1982. One of the research sites for networking software was the University of California at Berkeley. UCB was the center of development for the UNIX operating system for many years; the research done there also helped refine TCP/IP. In 1983, UCB released a version of UNIX that incorporated TCP/IP as an integral part of the operating system. TCP/IP became very popular because UNIX was widely used, especially as more sites connected to the growing ARPAnet.



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<P>When TCP/IP was designed, all of the services that had to be provided were considered. The best approach to implement all the services was to divide the different services into categories, such as end-user services (file transfer and remote logon), transport services (the way data is sent back and forth, invisible to the user), and network (how the data is packaged for transfer). A layered architecture was developed that isolates each set of services from the other.



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<P>A layered approach to designing the software requires more work initially, but has several important benefits. First, since each layer is independent of the others, changes to one service won't cause problems with other services. As new services are developed, they can be added without changing any other parts of the software system. Most importantly, layering makes it possible for a set of small, efficient programs to be developed for specific tasks, each of which can be independent of the others.



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<P>One condition needed to allow layered architecture to work properly is that each layer must know what is coming from the layer above or below. The layer may not care about the actual contents of the message, but it must know what to do with it. For example, if you are sending an e-mail message, you write the message and instruct the application layer to transmit it to the destination. The application layer sends the message down the layers until another layer sends it out across the network cable. Each layer handles the e-mail message but has no interest in the actual contents of the message.



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<P>To simplify this task, each layer adds a block of data at the front and the back of the message that indicates which layer is involved as well as all the other bits of information the other layers and the receiving machine needs to handle the message properly. The data within the message is ignored. This is called &quot;encapsulation,&quot; as each layer adds a capsule of information around the original data. Each layer performs its own encapsulation, adding header and trailer blocks to the message from the layer above. This results in several sets of headers and trailers by the time a message makes it to the network.



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



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<P>Every machine that is connected to a TCP/IP-based network has to be uniquely identified somehow. Without a unique identifier, the network wouldn't know how to get messages to your machine. If there were more than one machine with the same identifier, the network wouldn't know which one to send the message. To understand IP addresses, which uniquely identify each machine, it is useful to relate to the Internet's problem of identifying not only each machine on the huge Internet, but also each organization.



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<P>The Internet identifies networks by assigning an &quot;Internet address&quot; or, more properly, an &quot;IP address&quot; to each company or organization on the net. IP addresses are all 32 bits in length, and are broken into four 8-bit parts. This allows each part to have numbers ranging from 0 to 255. The four parts are combined in a notation called &quot;dotted quad,&quot; which means each 8-bit value is separated by a period. For example, &quot;255.255.255.255&quot; and &quot;147.120.3.28&quot; are both dotted quad IP addresses. When someone asks you for your network address, they usually mean your IP address.



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<P>Assigning IP addresses is not a simple matter of starting with the number 1 and counting upwards. Each machine (called a host) that can be attached to the Internet needs to be numbered. Even the dotted-quad 32-bit numbering scheme couldn't handle all the machines just by counting them. Instead, IP addresses work by identifying a network, then a machine on that network.



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<P>The IP address really consists of two parts: the network number and the host number within that network. By using two parts to the IP address, machines on different networks can have the same host number, but because the network number is different, the machines are uniquely identified. Without this type of scheme, the numbering would become unwieldy very quickly.



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<P>IP addresses are assigned on the basis of the size of the company or organization. If the company is small, there is no need for many machine identifiers within the network. On the other hand, a large corporation may have thousands of hosts. To provide for the maximum flexibility, IP addresses are assigned according to the size of the user, called &quot;Class A,&quot; &quot;Class B,&quot; or &quot;Class C.&quot; &quot;Class D&quot; and &quot;Class E&quot; have special purposes and are not used as part of the IP address numbering.



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<P>The three classes allow the IP address numbers to be assigned on the basis of company size. Since 32 bits is the total size allowable, the classes break the four 8-bit parts into the network and host identifiers depending on the class. One or more bits is reserved at the start of the 32-bit IP address to identify the type of class (hence, how other machines that analyze the IP address should decode the numbers). The three classes have the following breakdowns:



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<LI>Class A: network 7 bits; host 24 bits



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<LI>Class B: network 14 bits; host 16 bits



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<LI>Class C: network 21 bits; host 8 bits



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<P>A Class A address has only a 7-bit address for the network address but 24 bits for the host address. This allows over 16 million different host addresses, enough for the very largest organization. Of course, there can only be 128 of these Class A addresses, maximum.



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<P>Class B networks have 14 bits for the network and 16 bits for the host, allowing more Class B networks but fewer hosts. Still, 16 bits allow over 65,000 hosts. Finally, Class C IP addresses can have a maximum of 254 hosts (because the numbers 0 and 255 are reserved for each of the parts of the IP address) but have a lot of network IDs. The majority of networks are Class B and Class C.



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<P>It is possible to tell the type of class a company belongs to by the first number of the IP address. The rules for the first 8-bit number are:



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<LI>Class A addresses are between 0 and 127



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<LI>Class B addresses are between 128 and 191



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<LI>Class C addresses are between 192 and 223



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<P>So, if your host machine's IP address is 147.14.87.23, you know that your machine is on a Class B network, the network identifier is 147.14, and your host's unique number in that network is 87.23. If the IP address is 221.132.3.123, the machine is on a Class C network with a network identifier of 221.132.3, and a host number of 123.



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<P>Whenever a message is sent to a host anywhere on the Internet, the IP address is used to indicate its destination and the host that sent it. Luckily, because of another TCP/IP service called the Domain Name System, you don't have to keep track of all these IP addresses yourself.



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<FONT SIZE=5 COLOR="#FF0000"><B>The Domain Name System</B></FONT></CENTER></H3>



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<P>When a company or organization wants to use the Internet, they must decide whether they want to do it by directly attaching to the Internet system or whether they will use another company to supply the connection. Many companies choose to use another company, called a service provider, because it reduces the amount of equipment, administration, and costs involved.



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<P>If the company or organization wants to directly connect (and sometimes when they are using a service provider), they may want to have a unique identification for themselves. For example, ABC Corporation may want to have electronic mail through the Internet addressed to them as &quot;abc.com.&quot; The name helps identify the company or organization to the sender.



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<P>To obtain one of these unique identifiers called a &quot;domain name&quot;, the company or organization sends a request to the body that controls access to the Internet: the Network Information Center, or NIC. If the NIC approves the company's name, it is added to the Internet database. Domain names must be unique, to prevent confusion.



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<P>The part of the name that comes last (such as &quot;.com&quot;) is the domain identifier. These are the seven domain names established by the NIC:



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.arpa



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an ARPAnet-Internet identification</FONT>



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.com



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commercial company</FONT>



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.edu



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educational institution</FONT>



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