The Internet standards use a specific set of terms when referring to network elements and concepts related to TCP/IP networking. These terms provide a foundation for subsequent chapters illustrates the components of an IP network.
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Elements of an IP network
Common terms and concepts in TCP/IP are defined as follows:
• Node Any device, including routers and hosts, which runs an implementation of IP.
• Router A node that can forward IP packets not explicitly addressed to itself.
On an IPv6 network, a router also typically advertises its presence and host configuration information.
• Host A node that cannot forward IP packets not explicitly addressed to itself (a non-router). A host is typically the source and the destination of IP traffic. A host silently discards traffic that it receives but that is not explicitly addressed to itself.
• Upper-layer protocol A protocol above IP that uses IP as its transport.
Examples include Internet layer protocols such as the Internet Control Message Protocol (ICMP) and Transport layer protocols such as the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
• LAN segment A portion of a subnet consisting of a single medium that is bounded by bridges.
• Subnet One or more LAN segments that are bounded by routers and use the same IP address prefix. Other terms for subnet are network segment and link.
• Network Two or more subnets connected by routers. Another term for network is internet work.
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• Air A node connected to the same subnet as another node.• Interface The representation of a physical or logical attachment of a node to a subnet. An example of a physical interface is a network adapter.
• Address An identifier that can be used as the source or destination of IP packets and that is assigned at the Internet layer to an interface or set of interfaces.
• Packet The protocol data unit (PDU) that exists at the Internet layer and comprises an IP header and payload.
In a network, a computer is known as a host, sometimes a node, and every such host has a hostname. This name is unique throughout the network. Each machine is fitted with a network interface card that is connected by wire to the corresponding cards in other machines. All communication between hosts normally takes place through these network interfaces only.
Every TCP/IP network has an address that is used by external networks to direct their messages. Every host in the network has an address as well, and the combination of these two addresses forms the complete network address of the host. For instance, 192.168 (or strictly speaking, 192.168.0.0) could be the address of a network, and a host within the network could have the host address of 45.67. In that case, 192.168.45.67 represents the complete network address of the host. This address has to be unique not only within the network, but also to all connected networks. And, if the network is hooked up to the Internet, it has to be unique throughout the world.
When two or more networks are connected together, and uses the TCP/IP protocol for communication, we have an internet, popularly known as an intranet, which is the super – network of all networks. A local internet or intranet may easily be connected to the Internet which also uses the same protocol. Now many installations have several kinds of computers, including microcomputers, workstations, minicomputers, and mainframes. These computers are likely to be configured to perform specialized tasks. Although people are still likely to work with one specific computer, that computer will call on other systems on the net for specialized services. This has led to the “server/client” model of network services.
A server is a system that provides a specific service for the rest of the network. A client is another system that uses that service. (Note that the server and client need not be on different computers. They could be different programs running on the same computer.) Here are the kinds of servers typically present in a modern computer setup. Note that these computer services can all be provided within the framework of TCP/IP.
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• network file systems. A network file system provides the illusion that disks or other devices from one system are directly connected to other systems.
There is no need to use a special network utility to access a file on another system. Your computer simply thinks it has some extra disk drives. These extra “virtual” drives refer to the other system’s disks. This capability is useful for several different purposes. It lets you put large disks on a few computers, but still give others access to the disk space. Aside from the obvious economic benefits, this allows people working on several computers to share common files. It makes system maintenance and backup easier, because you don’t have to worry about updating and backing up copies on lots of different machines. A number of vendors now offer high-performance diskless computers. These computers have no disk drives at all. They are entirely dependent upon disks attached to common “file servers”.
• remote printing. This allows you to access printers on other computers as if they were directly attached to yours. (The most commonly used protocol is the remote lineprinter protocol from Berkeley Unix)
• remote execution. This is useful when you can do most of your work on a small computer, but a few tasks require the resources of a larger system.
There are a number of different kinds of remote execution. Some operate on a command by command basis. That is, you request that a specific command or set of commands should run on some specific computer. However there are also “remote procedure call” systems that allow a program to call a subroutine that will run on another computer.
• name servers. In large installations, there are a number of different collections of names that have to be managed. This includes users and their passwords, names and network addresses for computers, and accounts. It becomes very tedious to keep this data up to date on all of the computers. Thus the databases are kept on a small number of systems. Other systems access the data over the network.
• terminal servers. Many installations no longer connect terminals directly to computers. Instead they connect them to terminal servers. A terminal server is simply a small computer that only knows how to run telnet (or some other protocol to do remote login). If your terminal is connected to one of these, you simply type the name of a computer, and you are connected to it. Generally it is possible to have active connections to more than one computer at the same time. The terminal server will have provisions to switch between
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connections rapidly, and to notify you when output is waiting for another connection.• network-oriented window systems. Until recently, high-performance graphics programs had to execute on a computer that had a bit-mapped graphics screen directly attached to it. Network window systems allow a program to use a display on a different computer. Full-scale network window systems provide an interface that lets you distribute jobs to the systems that are best suited to handle them, but still give you a single graphically-based user interface.