Introduction to Computer Networks Notes

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Introduction to

Computer Networks

Introduction

All internet

activities require computers to access multiple networks and share

their resources.

Local Area Network (LAN)

is a network of users that share computer resources in a limited area.

Network Topologies

Protocol is the

set of rules established for users to exchange information.

Topology is the network

architecture used to interconnect the networking equipment.

Token Ring topology: a

“token” is placed in the data channel and circulates around the ring.

If a user wants to transmit, the computer waits until it has control of

the token.

Token Passing means each

station connected to the network ensures access for transmission of its

messages at regular or fixed time intervals.

Token Ring Hub: such a

device manages the passing of the token rather than relying on

individual computers to pass it, which improves the reliability of the

network.

Bus Topology: the

computers share the media of data transmission, a coaxial cable (called

ThinNet) is looped through each networking device to facilitate data

transfer.

The star topology is the

most common, twisted-pair cables with modular plugs are used to connect

the computers and other networking devices. At the center of a star

network is either a switch or a hub, this connects the network devices

and facilitates the transfer of data.

Hub receives data from a

network device and then broadcasts it to all connected devices, called

a multiport report repeater.

Switch can be used at the

center of a star network to minimize unnecessary data traffic and

isolate sections of the network.

Mesh

topology all networking devices are directly connected to each other,

this provides full redundancy in the network data paths but a cast.

The OSI Model

Layer 7:

Application

Support for

applications

HTTP, FTP, SMTP

Interacts with application

programs that incorporate a communication component such as your

internet browser and email. This layer is responsible for logging the

message in, interpreting the request, and determining what information

is needed to support the request.

Layer 6: Presentation

Protocol

conversion, data translation

ASCII, JPEG

Accepts and structures the

messages for the application. It translates the message from one code

to another if necessary. This layer is responsible for data compression

and encryption.

Layer 5: Session

Establishes,

manages, and terminates sessions

NFS, SQL

Provides the control

functions necessary to establish, manage, and terminate the connections

as required to satisfy the user request.

Layer 4: Transport

Ensures

error-free packets

TCP, UDP

Is concerned with message

integrity between source and destination. It also segments/reassembles

(the packets) and handles flow control.

Layer 3: Network

Provides routing

decisions

IP, IPX

Accepts

outgoing messages and combines messages or segments into packets,

adding a header that includes routing information. It acts as a network

controller.

Layer 2: Data Link

Provides for the

flow of data

MAC addresses

Handles error recovery.

Flow control (synchronization), and sequencing (which terminals are

sending and which are receiving). It is considered the “media access

control layer” and is where Media Access Control (MAC) addressing is

defined. The Ethernet 802.3 standard is defined in this area, which is

why the MAC address is sometimes called the Ethernet address.

Layer 1: Physical

Signals and

media

NICs,

twisted-pair cable, fiber

Provides the electrical

and mechanical connection to the network.

The Ethernet LAN

Introduction

The networking

protocol used in most modern computer networks is Ethernet, a carrier

sense multiple access with collision detection (CSMA/CD) protocol for

local area networks.

The information is

exchanged in a packet format. The packet provides grouping of the

information for transmission that includes the header, data, and

trailer.

The header consists of the

preamble, start frame delimiter, destination and source addresses, and

length/type field.

Next is the actual data

being transmitted, followed by the pad used to bring the total number

of bytes up to the minimum of 46 if the data field is less than 46

bytes.

The

last part is a 4-byte cyclic redundancy check (CRC) value used for

error checking.

The Ethernet Frame

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Preamble

An alternating

pattern of 1s and 0s used for synchronization.

Start Frame Delimiter

A binary 8-bit

sequence of 1 0 1 0 1 0 1 1 that indicates the start of the frame.

Destination MAC Address

Each computer

has an Ethernet network interface card (NIC) or network adapter that has

a unique media access control MAC address associated with it. The MAC

address is 6 bytes (12 hex characters) in length.

Source MAC Address

Each computer

has an Ethernet network interface card (NIC) or network adapter that has

a unique media access control MAC address associated with it. The MAC

address is 6 bytes (12 hex characters) in length.

Length/Type

An indication of

the number of bytes in the data field if this value is less than 1500.

If this number is greater than 1500, it indicates the type of data

format—for example, IP and IPX.

Data

The variable

length of data being transferred from the source to the destination.

Pad

A field used to

bring the total number of bytes up to the minimum of 46 if the data

field is less than 46 bytes.

Frame Check Sequence

A 4-byte CRC

value used for error detection. The CRC is performed on the bits from

the destination MAC address through the PAD fields. If an error is

detected, the frame is discarded.

Minimum length of the

Ethernet frame is 64 bytes from the destination MAC through frame check

sequence. The maximum length is 1518 bytes; 6byte for the destination

MAC; 6 bytes for the source MAC; 2 bytes for length/type; and 1500 bytes

for the data.

Network Interface

Card

Networked

devices, such as computers and network printers, each have an electronic

hardware interface to the LAN called a network interface card (NIC) or

integrated network port.

The NIC contains a unique

network address called the MAC Address.

MAC Address

MAC stands for

“media access control”.

The MAC address is 6 bytes

or 48 bits in length.

It is displayed in 12

hexadecimal digits. The first 6 digits are used to indicate the vendor

of the network interface, also called the organizationally unique

identifier (OUI), and the last 6 numbers form a unique value for each

NIC assigned by the vendor.

IEEE is the worldwide

source of registered OUIs.

The MAC address, also

called the Ethernet, physical, hardware, or adapter address, can be

obtained from computers operating under Microsoft windows by typing the

ipconfig /all command while in the command mode or at the MS-DOS prompt.

The following is an example of obtaining the MAC address for a computer

operating under windows 7, windows vista, or XP.

Ipconfig /all

MAC Address is

listed under Ethernet adapter local area connection: physical address.

The Media State: shows if

the computer is connected to a network device such as hub or switch.

IP Addressing

Internet

Protocol (IP) addressing provides a solution to worldwide addressing by

incorporating a unique address that identifies the computer’s local

network.

IP numbers are assigned by

Internet Assigned Numbers Authority (IANA)

IP addresses are classified

as either IPv4 or IPv6

The

IPv4 address is a 32-bit address that identifies on which network the

computer is located and differentiates the computer from all other

devices on the same network. It is divided into four 8-bit parts. The

format of the IP address is: x.x.x.x

Classes of IP Networks

Class A

Governments, very large networks

Exmp: 44.x.x.x

Max hosts: 224-2=16,777,214

0.0.0.0 to

127.255.255.255

Class B Midsize companies,

universities, and so on

Exmp:

128.123.x.x

Max hosts: 216-2=65,534

128.0.0.0 to

191.255.255.255

Class C Small Networks

Exmp:

192.168.1.x

Max Hosts: 28-2=254

192.0.0.0 to

223.255.255.255

Class D Reserved for

multicast groups

Exmp:

224.x.x.x

Max hosts: not applicable

224.0.0.0 to

239.255.255.255

Network Number: The portion

of the IP address that defines which network the IP packet is

originating from or being delivered to

Host Number: The portion of

the IP address that defines the location of the networking device

connected to the network; also called the host address.

Private Addresses: IP

addresses set aside for use in private intranets.

Intranet: an internal

internetwork that provides file and resource sharing.

IP Internetwork: A network

that uses IP addressing for identifying devices connected to the

network.

TCP/IP: Transmission

Control Protocol/Internet Protocol, the protocol suite used for

internetworks such as the internet.

The 3 address blocks for

private IP addresses are:

10.0.0.0 to

10.255.255.255

172.16.0.0 to

172.31.255.255

192.168.0.0

to 192.168.255.255

Home Networking

Wired Network:

uses cabling and connectors to establish the network connections.

Advantages

Faster network

data transfer speeds

Relatively inexpensive to

set up

Not susceptible to

outside interference.

Disadvantages

Cable

connections typically require specialized tools

Cable installation can be

labor-intensive and expensive

Wireless Network: uses

radio signals to establish the network connections.

Advantages

User Mobility

Simple installations

No cables.

Disadvantages

Security

Issues

Data transfer rates can

be slower than wired networks

Wi-Fi: Wi-Fi Alliance—an

organization that tests and certifies wireless equipment for compliance

with the 802.11x standards.

802.11x standards, is a

group of wireless standards developed under IEEE 802.11

IEEE: is the Institute of

Electrical and Electronics Engineers.

IEEE Wireless Standards

include:

802.11a

(Wireless-A): This standard can provide data transfer rates up to 54

Mbps and an operating range up to 75 feet. It operates at 5GHz.

802.11b (Wireless-B): This

standard can provide data transfer rates up to 11Mbps with ranges of

100-150 feet. It operates at 2.4GHz.

802.11g (Wireless-G): this

standard can provide data transfer rates up to 54 Mbps up to 150 feet.

It operates at 2.4GHz.

802.11n

(Wireless-N): This is the next generation of high-speed wireless

connectivity promising data transfer rates up to 4 x 802.11g speeds

(200+Mbps). It operates at 2.4GHz.

Components of a home

network can include:

Hub

Switch

Network Adapter

NIC

PC Card Adapter

Wireless-N adapter

USB type A connector

Router

Access Point

Wireless router

Broadband Modem/Gateway

Cable Modem

DSL Modem

Issues to be considered

when planning a home network:

Data Speed

Cost

Ease of Implementation

Appearance

Home Access

Public Access

Steps to insure

Connectivity

Step 1

Insure proper

connectivity on network device connecting to the ISP.

Step 2

Fixing basic

connection issues with ISP, reboot host computer and reboot router,

sometimes rebooting the Modem is needed.

Step 3

Verify

hardware cable or other connections have not been pulled loose.

Verify all wireless units

have a network connection.

Step 4

Verify the network settings.

Securing the Home Network

Change the

default factory passwords

Change the default SSID

Turn encryption on

Turn off the SSID

broadcast

Enable MAC address

filtering

Limit outside access

Network

address translation

Firewall protection

Establish a VPN

connection when transferring sensitive information.

IP Addressing in the Home

Network

The ISP issues

an IP address to the router and then the router will issue private IP

addresses to computers on the network using Network Address Translation

(NAT).

NAT: translates the

private IP address to a public address for routing over the internet.

Using Overloading, the NAT

translates the private address to the single public IP address assigned

by the ISP.

The NAT process tracks a

port number for the connection, this technique called Port Address

Translation (PAT).

The router stores the home

networks IP address and port number in a NAT lookup table.

The port number is used

when a data packet is returned to the home network. It identifies the

computer that established the connection, and then the router delivers

the data packet to the correct computer.

Assembling an Office LAN

Step 1

Document the

devices to be connected in the network and sketch out the proposed

network.

Each devices MAC and IP

addresses should be included in the network drawing documentation.

Step 2

Connect all the

networking devices using the corrected topology.

Using the Star Topology

there will be a switch or hub at the center. Connections from the hub

or switch to the networked devices is made with twisted-pair.

The media used for

transporting data in a modern computer network are either wireless,

twisted-pair, or fiber-optic cables.

Numerics

10Base2

10 Mbps over

coaxial cable up to 185 m, also called ThinkNet (Seldom used anymore)

10Base5

10Mbps over

coaxial cable up to 500 m, also called ThinkNet (seldom used anymore)

10BaseT

10Mbps over

twisted-pair

10BaseF

10Mbps over

multimode fiber-optic cable

10BaseFL

10Mbps over

850 nm multimode fiber-optic cable

100BaseT

100Mbps over

twisted-pair (also called Fast Ethernet)

100BaseFX

100Mbps over

Fiber

1000BaseT

1000Mbps over

twisted-pair

1000BaseFX

1000Mbps over

fiber

10GE

10GB Ethernet

Crossover: Transmit and

receive signal pairs are crossed to properly align the transmit signal

on one device with the receive signal on the other device.

Straight-through: Transmit

and receive signal pairs are aligned end-to-end, also called an uplink

port.

Link Light: Indicates the

transmit and receive signal pairs are aligned.

Link Integrity Test:

Protocol used to verify that a communication link between two Ethernet

devices has been established.

Link

Pulse: Sent by each of the connected devices via the twisted-pair

cables when data is not being transmitted to indicate that the link is

still up.

Step 3

Configure the

IP address settings on each computer.

Testing and Troubleshooting

a LAN

Verify the

computer are properly connected on the network.

Verify that you

have link lights on each switch port that is connected to a computer or

other networking device.

Use the ping command to

verify that the networking devices are communicating.

Ping uses

Internet Control Message Protocol (ICMP) echo requests and replies to

test that a device on the network is reachable.

[Ping (IP Address)] is

used to ping the IP address for a device.

If

the device at [IP Address] does not respond, the message “Request timed

out” is displayed.

To verify the IP

address of the computer you are working on use ipconfig /all