Introduction To Networking

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What is a computer Network

• Definition:

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Types of Networks

• Networks by components– P2P - Server-Based

• Networks by Size– LAN - MAN - WAN

• Networks by Topology– Star - Bus - Ring - Hierarchy -Mesh

• Networks by Media– Guided - Unguided

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Network Standards

• A number of organizations promote standardized components of data communications

• Examples:– ISOC– ANSI– ITU– IEEE– ISO

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Understanding the OSI Model

• In an effort to identify and standardize all the levels of communication needed in networking, ISO developed a networking model called the Open Systems Interconnection (OSI) reference model.

• The OSI reference model provides a universally accepted reference illustrating how data is transmitted on a network or between two or more networked devices.

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OSI Reference Model

• Foundation that brings continuity to LAN and WAN communications

• Product of two standards organizations:– ISO– ANSI

• Developed in 1974• Set of communication guidelines for

hardware and software design

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OSI Guidelines Specify…

• Methods to ensure that network transmissions are received correctly

• How network devices maintain a consistent rate of data flow

• How electronic data is represented on network media

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OSI Layers

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Understanding the OSI Model (Continued)Application Layer

• The Application layer of the OSI model is responsible for interfacing with application software, such as Web browsers or Web servers.

Presentation Layer• The Presentation layer receives requests for

files from the Application layer, and presents the requests to the Session layer.

• The Presentation layer reformats, compresses, or encrypts data as necessary.

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Understanding the OSI Model (Continued)

Session Layer

• The Session layer is responsible for establishing and maintaining a session between two networked stations or hosts.

• A host is any computer or other device on a network that has been assigned an IP address.

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Understanding the OSI Model (Continued)Transport Layer

• The Transport layer is responsible for error checking and requests retransmission of data if it detects errors.

• The Transport layer might or might not guarantee successful delivery of data.

Network Layer• The Network layer is responsible for

dividing a block of data into segments that are small enough to travel over a network.

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Understanding the OSI Model (Continued)

Network Layer (Continued)• Segments of data are called packets, data

packets, or datagrams and contain data, along with special identifying information in headers and trailers at the beginning and end of the packet.

Data Link Layer• The Data Link layer is responsible for receiving

packets of data from the Network layer and presenting them to the Physical layer for transport.

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Understanding the OSI Model (Continued)Physical Layer

• The OSI Physical layer is responsible for passing data packets on to the cabling or wireless media (whether the media be cabling or wireless).

• When software is permanently embedded on a hardware device, it is called firmware.

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Communicating Between Stacks

• OSI model provides standards for:– Communicating on a LAN– Communicating between LANs– Internetworking between LANs and WANs

and between WANs and WANs

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Peer Protocols

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Applying the OSI Model

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The OSI Model Applied to a TCP/IP Network

• On a TCP/IP network, TCP/IP is managed by the operating system and covers the first five layers of the OSI model.

• When a browser wants to access a Web server, it uses the address and port of the Web server to make a request to the operating system.

• This request takes the form of an Application Program Interface (API).

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The OSI Model Applied to a TCP/IP Network (Continued)

• In a TCP/IP network, the API call for a Web page from a Web server causes the operating system to generate an HTTP request.

• HTTP does not operate in the lower layers of the network model.

• Instead, it hands data over to TCP, which resides in the Transport layer.

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The OSI Model Applied to a TCP/IP Network (Continued)

• A frame provides information at the beginning of the data, called a header, and information at the end of the data, called a trailer.

• A checksum is a calculated value that can be compared to the value calculated by the receiving Data Link layer.

• This technique of calculating and comparing values is called a cyclical redundancy check (CRC).

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TCP/IP Protocols at Each Layer

• TCP/IP covers the first five layers of the

OSI model, and is included in an operating

system as a group of utilities called the

TCP/IP stack.

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Protocols at the Application, Presentation, and Session Layers

• The first three layers of the OSI model are handled by the protocol specific to the application using it and are best treated as a single group rather than unique layers.

• Web browsers, e-mail, chat rooms, and FTP software are examples of the applications that use the Internet.

• The language or protocol each of these applications uses is listed at the Application, Presentation, and Session layers.

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Protocols at the Transport Layer

• A TCP/IP network has two protocols that work at the Transport layer; one protocol guarantees delivery and the other does not.

• With TCP/IP, the protocol that guarantees delivery is TCP and the protocol that does not is UDP (User Datagram Protocol).

• TCP is used for client and server requests and responses.

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Protocols at the Transport Layer (Continued)

• Because TCP establishes a connection, it is called a connection-oriented protocol.

• UDP is a protocol that sends data without caring about whether the data is received.

• It does not establish a connection first; thus, it is called a connectionless protocol.

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Protocols at the Network Layer• TCP and UDP communicate with the Network

layer, which is sometimes called the Internet layer.

• Some of the other supporting protocols include ARP (Address Resolution Protocol), responsible for locating a host on a LAN; RARP (Reverse Address Resolution Protocol), responsible for discovering the Internet address of a host on a LAN; and ICMP (Internet Control Message Protocol), responsible for communicating problems with transmission to devices that need to know about these problems.

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Protocols at the Data Link and Physical Layers

• PPP (Point-to-Point Protocol) is used

over telephone lines, and allows a

computer to connect to a network using a

modem.

• PPP is the most popular protocol for

managing network transmission from one

modem to another.

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MAC Addresses

• MAC addresses function at the lowest

(Data Link) networking level.

• If a host does not know the MAC address

of another host on a local area network, it

uses the operating system to discover the

MAC address.

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IP Addresses

• All the protocols of the TCP/IP suite identify a device on the Internet or an intranet by its IP address.

• An IP address is 32 bits long, made up of 4 bytes separated by periods.

• Within an IP address, each of the four numbers separated by periods is called an octet.

• The first part of an IP address identifies the network, and the last part identifies the host.

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Classes of IP Addresses• IP addresses that can be used by companies

and individuals are divided into three classes: Class A, Class B, and Class C, based on the number of possible IP addresses in each network within each class.

• The group of IP addresses assigned to an organization are unique to all other IP addresses on the Internet and are available for use on the Internet.

• The IP addresses available to the Internet are called public IP addresses.

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Private IP Addresses• Private IP addresses are IP addresses

that are assigned by a network administrator for use on private intranets that are isolated from the Internet.

• The RFC 1918 recommends that the following IP addresses be used for private networks:– 10.0.0.0 through 10.255.255.255

– 172.16.0.0 through 172.31.255.255

– 192.168.0.0 through 192. 168. 255.255

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Dynamically Assigned IP Addresses

• Instead of IP addresses permanently being assigned to computers (called static IP addresses), an IP address is assigned for the current session only (called a dynamic IP address).

• Internet service providers (ISPs) are organizations through which individuals and businesses connect to the Internet.

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Network Address Translation

• If the hosts on a network using private IP addresses need to access the Internet, a problem arises because the private IP addresses are not allowed on the Internet.

• The solution is to use NAT (Network Address Translation), which uses a single public IP address to access the Internet on behalf of all hosts on the network using other IP addresses.

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Plans for New IP Addresses

• Because of an impending shortage of IP addresses, as well as some limitations in the current standards for IP, a new scheme of IP addresses called the IPv6 (IP version 6) standard is currently being developed and implemented.

• Current IP addresses using the current IPv4 (IP version 4) have 32 bits with eight bits in each of four octets.

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Plans for New IP Addresses (Continued)

• With the new system, each address segment can have 32 bits, for a total of 128 bits for the entire address.

• A disadvantage of IPv6 is the fact that so much software used on the Internet would become outdated because current software is designed to hold 32-bit IP addresses and, with the new system, this number would no longer be sufficient.

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Ports• A port is a number used to address

software or services running on a computer.

• A host computer might have several services running on it.

• Each server running on the host is assigned a port.

• The port is written at the end of the IP address, separated from the IP address with a colon—like this: 169.49.209.19:80.

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Tying It All Together: How Data Travels on the Network

• The Web browser wants to make a request to a Web server, and processes the request using an API call.

• The API process packages the data using HTTP format, which includes an HTTP header, and addresses it to an IP address and port 80, which is the default port for a Web server.

• HTTP delivers the package to TCP, giving the destination IP address and port.

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Tying It All Together: How Data Travels on the Network (Continued)• TCP guarantees delivery, and attempts to make

a connection to the destination IP address and port using the three-way handshake.

• With the first attempt that TCP makes to communicate with the remote host, the first packet reaches the Network layer and IP uses ARP to resolve the IP address to a MAC address.

• When a packet reaches a hub on the Ethernet LAN, the hub replicates the packet and sends it to every device to which it is connected.

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Tying It All Together: How Data Travels on the Network (Continued)• When the IP layer receives a broadcast

packet, it decides if it is appropriate to respond.

• After it knows the MAC address, it can send the first packet to make the connection.

• It prepares the data by attaching a TCP header in front of the data.