1.3.2 Networks

A set of rules to govern communication across a network. Examples: TCP/IP | POP | SMTP | FTP

• Allows networks to communication • Ensures all devices follow the same rules • Ensures devices interpret signals in the same way

An agreed way of doing things Examples: USB | HTML | MP3

Standards make it easier for people to build programs and software that work on different systems in different countries.

Domain Name System (DNS) Used to convert a given URL to a corresponding IP address.

• URL (www.google.co.uk) is entered and sent to the DNS resolver • DNS servers map this domain to an IP address, by looking up in a lookup table/cache. • If the DNS server cannot find it, then it passes the request (recursively) to the top-level domain name server. • If the IP address is found it updates the DNS servers that did not know it. • If it is not found, then it will return an error • The original DNS Server sends the IP address to the browser so it can retrieve the website from the server on which it is hosted.

A logical identifier that uniquely identifies a device on a network (e.g. 192.168.0.1)

A physical identifier that uniquely identifies a device on a network * Stored in hexadecimal

Application Transport Internet Link

• Sits at the top of the stack • Protocol to be used is decided based on the application (HTTPS for browser | POP for email etc..) • Passes on to the transport layer to send • Gets data from the transport layer when receiving data • If using HTTPS - Encrypts and decrypts messages

• Uses the Transmission Control Protocol (TCP) to establish an end-to-end connection • Splits the data into packets • Adds the packet number, total, and port. • Agrees comminucation settings • Agrees the size of the packets they will send/receive

Routers operate on this layer Adds the IP address to the packet (to/from)

• Source and destination MAC addresses added to the packet header • Receives data from the internet layer to send • Passes and receives data across the network • Passes data back up to the internet layer when receiving

• Applies protocols in order - one after the other • Each layer does a specific job • Groups similar protocols together • Each layer only communicates with adjacent layers (+) Allows layers to be upgraded without affecting others. (+) Hardware / Software can be manufactured to fit into one specific layer (+) Allows for layers from different providers to be used interchangeably.

LAN – Local Area Network • A group of devices connected over a small geographical area. • The equipment is owned by the network owner. • The range could include a building or campus.

WAN – Wide Area Network • A group of devices connected over a large geographical area • Often uses third-party communication channels. • Range could include the entire world

Creates a direct link between two devices for the duration of the communication. The public telephone system is an example. (-) The connection is set up for the entire duration of the message so bandwidth is wasted during the periods when no data is being sent. (-) The two devices must transmit and receive data at the same rate (+) Data packets arrive in the same order that they are sent, simplifying the process of reconstructing the message at the recipient end.

• Data is split into equal-sized packets. • Each packet has a header of information (including the destination address and the place of the packet in the complete message) • Each packet is placed on the network, each may travel by a different route, meaning they may arrive at different times. • At each node on the network, the destination address is read, and the best route is found. • Packets need to be reordered at the destination.

• Transmission is safer from interception because it is impossible to intercept all the packets as they use different routes. • Very efficient use of the network as it does not tie up a part of the network. • If there is an error then only a small, identifiable, part of the data is affected, and this can be retransmitted easily.

A collection of two or more computers connected in order to exchange information and share resources.

• File and Data Sharing • Resource Sharing (Printers etc..) • Data Protection and Redundancy • Ease of Administration - central maintenance • Distributing Computing Power • Internal Communications • Share internet connection • Can monitor user activity

Client computers connect to and are controlled by a central server The server provides access to a resource/service (depending on the user)

(+) Centrally administered in one location, including pushing out S/W updates and security patches. (+) One location to back up (file server) (+) Shared resources such as printers are not tied to a single computer (-) Has a central point of failure (server) (-) Can be expensive to maintain/set up (e.g. cabling costs, specialist staff.)

(+) Has no central point of failure (+) Cheap to set up and run. (-) Multiple locations to back up (-) Devices must maintain their own security. (-) Shared resources are lost when the connected computer is offline