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

9/1/2016 4:50 PM - Screen Clipping

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

	Creado por Justin Bullock hace casi 8 años

(Introduction to Computer Networks) Network and Security: Foundations Apunte sobre Introduction to Computer Networks Notes, creado por Justin Bullock el 11/10/2016.