1. LAN (Local Area Networks)Networks infrastructures can vary greatly in terms of:
The size of the area covered
The number of users connected
The number and types of services available
An individual network usually spans a single geographical area, providing services and applications to people within a common organizational structure, such as a single business, campus or region. This type of network is called a Local Area Network (LAN). A LAN is usually administered by a single organization. The administrative control that governs the security and access control policies are enforced on the network level.
2. WAN (Wide Area Networks)When a company or organization has locations that are separated by large geographical distances, it may be necessary to use a telecommunications service provider (TSP) to interconnect the LANs at the different locations. Telecommunications service providers operate large regional networks that can span long distances. Traditionally, TSPs transported voice and data communications on separate networks. Increasingly, these providers are offering converged information network services to their subscribers.
Individual organizations usually lease connections through a telecommunications service provider network. These networks that connect LANs in geographically separated locations are referred to as Wide Area Networks (WANs). Although the organization maintains all of the policies and administration of the LANs at both ends of the connection, the policies within the communications service provider network are controlled by the TSP.
WANs use specifically designed network devices to make the interconnections between LANs. Because of the importance of these devices to the network, configuring, installing and maintaining these devices are skills that are integral to the function of an organization's network.
LANs and WANs are very useful to individual organizations. They connect the users within the organization. They allow many forms of communication including exchange e-mails, corporate training, and other resource sharing.
3. The Internet - A Network of NetworksAlthough there are benefits to using a LAN or WAN, most of us need to communicate with a resource on another network, outside of our local organization.
Examples of this type of communication include:
Sending an e-mail to a friend in another country
Accessing news or products on a website
Getting a file from a neighbor's computer
Instant messaging with a relative in another city
Following a favorite sporting team's performance on a cell phone
InternetworkA global mesh of interconnected networks (internetworks) meets these human communication needs. Some of these interconnected networks are owned by large public and private organizations, such as government agencies or industrial enterprises, and are reserved for their exclusive use. The most well-known and widely used publicly-accessible internetwork is the Internet.
The Internet is created by the interconnection of networks belonging to Internet Service Providers (ISPs). These ISP networks connect to each other to provide access for millions of users all over the world. Ensuring effective communication across this diverse infrastructure requires the application of consistent and commonly recognized technologies and protocols as well as the cooperation of many network administration agencies.
IntranetThe term intranet is often used to refer to a private connection of LANs and WANs that belongs to an organization, and is designed to be accessible only by the organization's members, employees, or others with authorization.
Note: The following terms may be interchangeable: internetwork, data network, and network. A connection of two or more data networks forms an internetwork - a network of networks. It is also common to refer to an internetwork as a data network - or simply as a network - when considering communications at a high level. The usage of terms depends on the context at the time and terms may often be interchanged.
The Interaction of Protocols:An example of the use of a protocol suite in network communications is the interaction between a web server and a web browser. This interaction uses a number of protocols and standards in the process of exchanging information between them. The different protocols work together to ensure that the messages are received and understood by both parties. Examples of these protocols are:
Application Protocol:Hypertext Transfer Protocol (HTTP) is a common protocol that governs the way that a web server and a web client interact. HTTP defines the content and formatting of the requests and responses exchanged between the client and server. Both the client and the web server software implement HTTP as part of the application. The HTTP protocol relies on other protocols to govern how the messages are transported between client and server
Transport Protocol: Transmission Control Protocol (TCP) is the transport protocol that manages the individual conversations between web servers and web clients. TCP divides the HTTP messages into smaller pieces, called segments, to be sent to the destination client. It is also responsible for controlling the size and rate at which messages are exchanged between the server and the client.
Internetwork Protocol: The most common internetwork protocol is Internet Protocol (IP). IP is responsible for taking the formatted segments from TCP, encapsulating them into packets, assigning the appropriate addresses, and selecting the best path to the destination host.
Network Access Protocols:Network access protocols describe two primary functions, data link management and the physical transmission of data on the media. Data-link management protocols take the packets from IP and format them to be transmitted over the media. The standards and protocols for the physical media govern how the signals are sent over the media and how they are interpreted by the receiving clients. Transceivers on the network interface cards implement the appropriate standards for the media that is being used.
The Peer-to-Peer ModelIn addition to the client/server model for networking, there is also a peer-to-peer model. Peer-to-peer networking involves two distinct forms: peer-to-peer network design and peer-to-peer applications (P2P). Both forms have similar features but in practice work very differently.
Peer-to-Peer NetworksIn a peer-to-peer network, two or more computers are connected via a network and can share resources (such as printers and files) without having a dedicated server. Every connected end device (known as a peer) can function as either a server or a client. One computer might assume the role of server for one transaction while simultaneously serving as a client for another. The roles of client and server are set on a per request basis.
A simple home network with two connected computers sharing a printer is an example of a peer-to-peer network. Each person can set his or her computer to share files, enable networked games, or share an Internet connection. Another example of peer-to-peer network functionality is two computers connected to a large network that use software applications to share resources between one another through the network.
Unlike the client/server model, which uses dedicated servers, peer-to-peer networks decentralize the resources on a network. Instead of locating information to be shared on dedicated servers, information can be located anywhere on any connected device. Most of the current operating systems support file and print sharing without requiring additional server software. Because peer-to-peer networks usually do not use centralized user accounts, permissions, or monitors, it is difficult to enforce security and access policies in networks containing more than just a few computers. User accounts and access rights must be set individually on each peer device.
Peer-to-Peer ApplicationsA peer-to-peer application (P2P), unlike a peer-to-peer network, allows a device to act as both a client and a server within the same communication. In this model, every client is a server and every server a client. Both can initiate a communication and are considered equal in the communication process. However, peer-to-peer applications require that each end device provide a user interface and run a background service. When you launch a specific peer-to-peer application it invokes the required user interface and background services. After that the devices can communicate directly.
Some P2P applications use a hybrid system where resource sharing is decentralized but the indexes that point to resource locations are stored in a centralized directory. In a hybrid system, each peer accesses an index server to get the location of a resource stored on another peer. The index server can also help connect two peers, but once connected, the communication takes place between the two peers without additional communication to the index server.
Peer-to-peer applications can be used on peer-to-peer networks, client/server networks, and across the Internet.
DNSIn data networks, devices are labeled with numeric IP addresses, so that they can participate in sending and receiving messages over the network. However, most people have a hard time remembering this numeric address. Hence, domain names were created to convert the numeric address into a simple, recognizable name.
On the Internet these domain names, such as www.cisco.com, are much easier for people to remember than 198.133.219.25, which is the actual numeric address for this server. Also, if Cisco decides to change the numeric address, it is transparent to the user, since the domain name will remain www.cisco.com. The new address will simply be linked to the existing domain name and connectivity is maintained. When networks were small, it was a simple task to maintain the mapping between domain names and the addresses they represented. However, as networks began to grow and the number of devices increased, this manual system became unworkable.
The Domain Name System (DNS) was created for domain name to address resolution for these networks. DNS uses a distributed set of servers to resolve the names associated with these numbered addresses.
The DNS protocol defines an automated service that matches resource names with the required numeric network address. It includes the format for queries, responses, and data formats. DNS protocol communications use a single format called a message. This message format is used for all types of client queries and server responses, error messages, and the transfer of resource record information between servers.
DNS is a client/server service; however, it differs from the other client/server services that we are examining. While other services use a client that is an application (such as web browser, e-mail client), the DNS client runs as a service itself. The DNS client, sometimes called the DNS resolver, supports name resolution for our other network applications and other services that need it.
When configuring a network device, we generally provide one or more DNS Server addresses that the DNS client can use for name resolution. Usually the Internet service provider provides the addresses to use for the DNS servers. When a user's application requests to connect to a remote device by name, the requesting DNS client queries one of these name servers to resolve the name to a numeric address.
Computer operating systems also have a utility called nslookup that allows the user to manually query the name servers to resolve a given host name. This utility can also be used to troubleshoot name resolution issues and to verify the current status of the name servers.
In the figure, when the nslookup is issued, the default DNS server configured for your host is displayed. In this example, the DNS server is dns-sjk.cisco.com which has an address of 171.68.226.120.
We then can type the name of a host or domain for which we wish to get the address. In the first query in the figure, a query is made for www.cisco.com. The responding name server provides the address of 198.133.219.25.
The queries shown in the figure are only simple tests. The nslookup has many options available for extensive testing and verification of the DNS process.
E-mail Server Processes - MTA and MDAThe e-mail server operates two separate processes:
Mail Transfer Agent (MTA)
Mail Delivery Agent (MDA)
The Mail Transfer Agent (MTA) process is used to forward e-mail. As shown in the figure, the MTA receives messages from the MUA or from another MTA on another e-mail server. Based on the message header, it determines how a message has to be forwarded to reach its destination. If the mail is addressed to a user whose mailbox is on the local server, the mail is passed to the MDA. If the mail is for a user not on the local server, the MTA routes the e-mail to the MTA on the appropriate server.
TELNETTelnet was developed to meet that need. Telnet dates back to the early 1970s and is among the oldest of the Application layer protocols and services in the TCP/IP suite. Telnet provides a standard method of emulating text-based terminal devices over the data network. Both the protocol itself and the client software that implements the protocol are commonly referred to as Telnet.
Appropriately enough, a connection using Telnet is called a Virtual Terminal (VTY) session, or connection. Rather than using a physical device to connect to the server, Telnet uses software to create a virtual device that provides the same features of a terminal session with access to the server command line interface (CLI).
To support Telnet client connections, the server runs a service called the Telnet daemon. A virtual terminal connection is established from an end device using a Telnet client application. Most operating systems include an Application layer Telnet client. On a Microsoft Windows PC, Telnet can be run from the command prompt. Other common terminal applications that run as Telnet clients are HyperTerminal, Minicom, and TeraTerm.
Once a Telnet connection is established, users can perform any authorized function on the server, just as if they were using a command line session on the server itself. If authorized, they can start and stop processes, configure the device, and even shut down the system.
IPInterrupt Process (IP) - Suspends, interrupts, aborts, or terminates the process to which the Virtual Terminal is connected. For example, if a user started a program on the Telnet server via the VTY, he or she could send an IP command to stop the program.
While the Telnet protocol supports user authentication, it does not support the transport of encrypted data. All data exchanged during a Telnet sessions is transported as plain text across the network. This means that the data can be intercepted and easily understood.
If security is a concern, the Secure Shell (SSH) protocol offers an alternate and secure method for server access. SSH provides the structure for secure remote login and other secure network services. It also provides stronger authentication than Telnet and supports the transport of session data using encryption. As a best practice, network professionals should always use SSH in place of Telnet, whenever possible.
User Datagram Protocol (UDP)UDP is a simple, connectionless protocol, described in RFC 768. It has the advantage of providing for low overhead data delivery. The pieces of communication in UDP are called datagrams. These datagrams are sent as "best effort" by this Transport layer protocol.
Applications that use UDP include:
Domain Name System (DNS)
Video Streaming
Voice over IP (VoIP)
Transmission Control Protocol (TCP)TCP is a connection-oriented protocol, described in RFC 793. TCP incurs additional overhead to gain functions. Additional functions specified by TCP are the same order delivery, reliable delivery, and flow control. Each TCP segment has 20 bytes of overhead in the header encapsulating the Application layer data, whereas each UDP segment only has 8 bytes of overhead. See the figure for a comparison.
Applications that use TCP are:
Web Browsers
E-mail
File Transfers
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