Lecture 2 -_understanding_networks_with_presenter_notes

LECTURE 2: UNDERSTANDING NETWORKS

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Network Terminology Network Components Operating Systems on the

Network The OSI Model and

Networking Addressing on a Network How Data Travels on the

Network: an Example

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Network Adapter ◦ Usually an expansion card called NIC (Network

Interface Card) Network Protocol

◦ A set of rules for network communications. Examples: OS protocols: TCP/IP, NetBEUI Hardware protocols: Ethernet, Token Ring

Segments, Packets, Datagrams ◦ Small “chunks” of data that travel on network◦ In the network media, the packets travel in frames,

which carry each data packet with a header and trailer to identify each packet

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Hardware:◦ Network Interface

Card (NIC) ◦ Servers◦ Workstations◦ Hub, MAU,

Concentrator, Switch◦ Connectors◦ Cable◦ UPS

Software:◦ Operating Systems◦ Applications◦ Middleware

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Communications between computers require network OS(es) and network protocols.

Functions of OS in a computer: ◦ manage applications, hardware, and

connection to the network.◦ enabling the computers on the local area

network (LAN) to share their resources Protocols specify network functionality.

◦ Using the same set of network protocols, different devices can communicate.

◦ The selected network protocol must be supported by every operating system on the network.

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How resources are shared:◦ peer-to-peer model: several computers

using different operating systems in a small business or home can be connected to form a small LAN.

◦ client/server model, a.k.a. domain model, a server is used to control which resources on the LAN are shared, and who can access these resources.

Network OSes are used to manage resource sharing through mechanisms such as◦ user account: a collection of all of the

information that pertains to a user on a computer

◦ authentication, validation, or logging on: The process of entering a correct user ID and password to gain access to a computer

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Windows 98 and Windows Me Windows NT Workstation, Windows

2000 Professional, Windows XP Linux Macintosh OSes e.g. Mac OS X

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can be installed on a server and used to manage network resources, including user accounts, printers, and file sharing across the LAN.

Windows NT Server, Windows NT Enterprise Server

Windows 2000 Windows Server 2003 Novell NetWare UNIX, Linux

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Some criteria to consider when selecting a NOS are as follows:◦ Reliability◦ Performance◦ Adaptability◦ Affordability◦ Security◦ Scalability◦ Ease of use and ease of installation

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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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Physical Layer Physical layer is responsible for: Passing data

packets on to the cabling or wireless media (whether the media be cabling or wireless).

Data Link Layer Data Link layer is responsible for: Receiving

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

E.g: NIC (with firmware) When software is permanently embedded on a

hardware device, it is called firmware.

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Network Layer Network layer is responsible for:

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

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.

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Transport Layer 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 (depends on the transport protocol used).

Session Layer 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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Presentation Layer Presentation layer is responsible for:

receiving 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.

Application Layer The Application layer of the OSI model is

responsible for interfacing with application software, such as Web browsers or Web servers.

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The Web browser wants to make a request to a Web server, and processes the request using an API (Application Program Interface )call to the OS.

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.

19Fig 5-33 An application asks the OS to do something using an API call

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

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TCP hands the data off to IP, which resides in the network layer and is also managed by the OS.

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The data with its header information is a long stream of bytes. IP breaks it into individual packets. IP adds its own IP header that contains its own IP address (source) and the server’s IP address (destination). IP passes the packets off to the hardware (NIC).

22Fig 5-36 IP divides data into packets, which are then released to the network

On the NIC, each packet is given information at its beginning and end in the form of frame. A checksum is calculated using a technique called cyclical redundancy check (CRC). Then each frame is sent off to the Ethernet cable.

At the destination, the checksum is verified and passed to the IP layer to be regrouped.

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IP then passes the reassembled data and header stream on to the TCP layer who acknowledges the TCP layer on the client.

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The HTTP server, listening at port 80, receives the data and passes it on to the Web server.

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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.

Figure 5-41 shows the four major

groupings of the OSI model as applied

to TCP/IP networks.

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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.

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

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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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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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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. 33

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 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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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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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 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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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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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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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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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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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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