Networks

CH of OCR A Level Computer Science

Internet Protocol Addressing System

Each device on a network needs to be uniquely identified so that data can be sent to the correct destination.

Data Packet

A segment of data that needs to be sent, often referred to as the payload.

Header → Sender and Recipient IP protoco. Packet number x of y.

Trailer → End of the packet flag, checksum.

Buffer Overflow

Occurs when a program accidentally writes data to a location too small to hande it.

Packet Filtering

Packets of data are inspected by the firewall to check which port they are attempting to access.

Indexing

When a search engine keeps a record of the resources it locates.

Index

The search engine index contains entries for trillions of web pages, the size of googles index is estimated at over 100 petabytes.

Domain Name System

Dedicated computers with an index of domain names and their corresponding IP address.

Uniform Resource Locator (URL)

Used to specify means of accessing a resource accross a network and its location.

HTTPS:/ = Protocol

website.com = Domain

/glossaries = resource/path

Search Engine

Systems that locate resources on the WWW, search engines make it easier to find the relevant resources in an efficient way.

Web Crawlers

Internet bots that continuously crawl the web to discover and record publically available web pages.

Information Stored by Web Crawlers

> Resource URL

> Contents of the resource

> Last time updated

> Quality of the resource

and more…

Encryption

Encoding a plain text message so that it cannot be deciphered unless you have a numerical key to decrypt it.

Backbone

Dedicated physical connectionsthat connect several large networks at various points across the globe. Usually controlled by the internet service provider (ISP)

Internet

The internet is a network of interconnected networks.

Local Area Network (LAN)

Two or more devices connected in a small geographical area.

Wide Area Network (WAN)

A large computer network in a wide geographical area.

Wireless Access Point (WAP)

A wireless access point for a network, it requires a connection to a router.

Router

The ‘bridge’ between LAN and WAN

Packet Size

Deliberately kept small (500 - 1500 Bytes), ensures that individual packets dont take excessive time to transfer.

Firewalls

Software and hardware that controls access to and from a network.

Proxy Servers

Makes a request on behalf of your own computer, hiding the true request IP addresses from the recipient.

Can be used for Anonymous Surfing, Filtering Content, and Providing a cache.

SQL Injection

A malicious user can enter SQL commands via an online database form to change the processing of your device.

Input Steralisation

Special characters are turned into text characters to ensure any added code becomes invalid.

Malware

Annoys the user or damages their data.

Virus

Self-replicating, embeds itself in the programs or files. Requires a users interaction to work.

Worms

Stand alone program that requires no user aid and exploits the systems vulnerabilities.

Trojans

Malware that masquerades as innocuous or useful applications, they cannot self-replicate and acts to openm backdoors in a computer.

Phishing

Typically emails, manipulates a user into giving away personal data.

Wireless Network Interface Card (NIC)

A station consists of a computer and a NIC, stations share a radio frequency channel.

Bus Topology

Arrangement where nodes are connected in a daisy chain by a single computer network.

1. It is slow

2. Does not scale well

3. Contains a terminator

4. Cheaper

5. Restricted by Cable length

Star Topology

Arrangement where nodes are connected by a common connecton node.

1. Easy to fix/isolate issues

2. Better performance and scalability

3. Central device is a point of failure

4. Can be expensive to set up

TCP/IP Stack

A set of rules used in turn, to format a message so it can be sent over a network.

Application Layer → Used to provide service for applications that want to communicate across networks. It seperates control instructions and data, using FTP. it has two different ports.

Transport Layer → Uses the transmission control protocol (TCP) to establish an end-to-end connection with the recipients computer. Splits data into packets, numbering them.

Internet/Network Layer → Uses IP to address packets with source and destination IP addresses. Labels the packets then sends them on.

Link Layer → Finds next locations MAC address. Sending over network connection operating the physical connection between devices.

Port Numbers

A port is used to alert a specific application to a network message.

(Port 20) File Transfer Protocol Data

Application level protocol used to move files accross a network.

(Port 21) File Transfer Protocol Control

controls the FTP Data.

(Port 22)

Secure Shell (SSH) Remote Login

(Port 443)

HyperText Transfer Protocol Secure (HTTPS)

(Port 23)

Telnet Unencrypted Remote Login

(Port 80/8080)

HyperText Transfer Protocol (HTTP)

(Port 143) Interim Mail Access Protocol (IMAP)

Manages emails on a server so multiple clients can access the same email account in Synchronicity.

(Port 110) Post Office Protocol V3 (POP3)

Downloads emails stored on a remote server to a local client.

(Port 25) Simple Mail Transfer Protocol (SMTP)

Used to send/forward emails between mail servers to their destination.

Client Server

Features:

1. User IDs, passwords and access levels are centrally controlled

2. Used in many large to small organisations.

3. Can be expensive to set up and maintain.

4. Backups usually are automated.

5. No access to other users files.

Peer-To-Peer Servers

Features:

1. Files and programs stored on individual computers.

2. Suitable for a home network.

3. Cheap to set up and maintain.

4. Each computer on the network acts as a client server.

5. Can be used for sharing of music and streaming coverage of live events.

Client-Server Networks

Central Server to manage security, some files held on a central server, some processing tasks performed by the server.

Peer-To-Peer Networks

No central server, each device is connected to each other.

Client-Server Functions

1. Initial Validation

2. Web Page Interactivity

3. Manipulating Interface Elements

4. Applying Styles

5. Reduces Server Load

6. Reduces Web Traffic

Server Processing Functions

1. Database Queries

2. Encoding Data to Readable HTML

3. Updating the Database

4. Calculations

5. Further Validation

6. Keeps Data Secure

Meta Tags

Describes the contents of a web page, web develepors can place meta tags inside HTML pages to make the pages more likely to be found.

Page Rank

Developed to list search result in the orfer and rank of usefulness and relevance.

PageRank Algorithm

PR(A)=(1-d)+d(PR(Ti)/C(Ti))+…+PR(Tn)/c(Tn)

PR(A)

Page Rank of Page A

d

Damping Factor, Usually 0.85

PR(Ti)

Page Rank of pages Ti which link to page A.

C(Ti)

The number of outbound links on page Ti

PageRank Factors

1. Domain Name → Relevance to the search item.

2. Frequency of search term in web page.

3. Age of web page.

4. Frequency of page updates.

5. Magnitude of content updates.

6. Keywords in <H1> tags,