Networks Flashcards
What is a network?
Connected devices that can communicate, share resources and data
What are protocols?
Rules and standards that govern how networks should function and communicate
What is a topology?
Theoretical arrangement of the comments of a networks. Actual arrangement is influenced by physical factors and cost
Draw a ring, bus, star and mesh topology and explain the +/- of each
Picture can be found in photo Library
Star: + no data collisions, fast, robust, cheap +hub can be node switch etc, can be split into segments
Ring: +few collisions due to one way traffic, fast, one cable (cheap), passes through NIC of each node
- if one node fails the whole network could
Mesh: +Decentralised, wired or wireless, no single point of failure, no need for Internet and directly communicate, Good for communication
- lots of expensive cable
Bus: + simple, cheap (one cable)
- lots of collisions, if the cable breaks so does the network, slow with heavy traffic
What is a LAN?
Local Area Network. Nodes connected are geographically close
Benefits/properties of LAN?
- organisation owns infrastructure
- allows communication
- Allows peripherals to be shared
- Allows data to be shared
- Can log on from any connected computer
- easy upgrade and virus scans
- Can run a program simultaneously on multiple devices
1) What is a WAN?
2) What is a MAN?
1) Wide Area Network - result of joining multiple LANs via satellite, fibre optics cables, telephone cables, or a mixture - nodes a re geographically distant
2) Metropolitan Area Networks, provide WAN services in a city
Properties of WAN?
- infrastructure owned by 3rd party
- Same properties as WAN but it allows it at a much further distance
What is a SAN? Give an example
Storage Area Network - used for large scale storage of data in data centres
Email servers, databases and high usage file servers
What Is a PAN?
Personal Area Network - Used for transmission among devices such as computers, smartphones and tablets - can also be used to connect to high level networks or Internet
Advantages and disadvantages of cloud storage?
\+Data moves off site \+Maintained by third party \+Cheaper \+scalability -Loss of control -Security issues (?)
What are the properties of a peer-peer network?
- All nodes have = status
- All share files and peripherals
- each node can be accessed by another peer
- speed decreases due to lots of collisions
- Implemented over Internet
- Used for legal and illegal file sharing
What are the properties of a client-server network?
- Traditional architecture
- Dedicated high spec machine is server
- centralised data storage
- Allows processing of shared files, printing, Internet access
- manages security
- Clients request access to services from the servers (file server, web server, printer server etc)
What are data centres?
Multiple servers stacked together
What are virtualised servers? +/-
One server is split into multiple “virtual servers” using cloud computing.
+Efficient
+Lower energy consumption
What is the OSI and why was it created?
- Open systems interconnection evolved to create a standard of communication between different device types. Describes protocols that allow comps with different architectures to be linked together so that they can share data and communicate with each other
- just a model
Describe the 7 layers of the OSI
Application - Network aware applications. what the user interacts with (email, web browsing etc)
Presentation - converts data between formats required for different applications +transmission over network. Eg: Compression, encryption and translation
Session - Controls communication between connected devices. Starts connections, manages them and terminates them. User login rights, file/folder perms
Transport - Guarantees end to end delivery of data. Make sure data is present and that it gets to destination
Network - Transmits and routes data via shortest possible path across network. Shortest time not distance
Data Link - decides whose turn it is to send/receive data
Physical - Physical properties of network (cable, bit encoding, transfer rates etc)
What is the TCP/IP stack?
- A suite of protocols that describes how data is sent over a network
- Not just a model
Four layers of the TCP/IP stack?
Application - Production, communication and reception of data, makes sure data in correct format
Transport - Session layer + responsible for providing constant flow
Internet - provides links to transmit datagrams across different networks
Link - Passing Datagrams to local and physical network. Allows network hardware Independence therefore allowing it to operate over any transmission medium such as copper wire, optical fibre etc
What does PDU mean?
- Protocol Data Unit - Term used to describe data on any given layer of the stack
- Application = data
- Transport = Segment/Datagram
- Internet = Datagram
- Network access layer = frame/bits
What term is used to describe data on all levels instead of there associated PDU?
Packet
How do each of the four layers identify the source and destination address of data?
Application - just data no addressing needed
Transport - Data broken into segments that use ports to identify services
Internet - Devices have unique IP address, IP datagrams use IP address to reach correct destination
Link - Ethernet splits packets into frames. Frames use MAC address of device to reach it
What devices are used on each layer of the TCP/IP stack and OSI and what do they do?
- Physical = devices that extend the physical network (repeaters/transceivers)
- Data Link = Devices that make decisions based on MAC address of sender/recipient (bridges forwards frames to destination MAC address and switches which are bridges with multiple ports)
- Internet/Network = devices examine data packets and make decisions based on IP address of sender/recipient (Router examines contents and finds appropriate network to route data to based on IP)
- Transport = devices examine segments and make decision base on port number. Firewalls use port numbers
- Application = devices examine contents of data and make decision based on content (Checking email for spam). Application layer gateway
What is a Datagram?
A self contained unit of data that is the basic unit of data used in transmission. Allows data to be treated independently from one another which increases reliability
Negatives of Datagram?
- Sent without a pre defined route meaning no guarantee that it will reach destination
- No confirmation of successful transmission
- Order of sending/receiving not considered
What is a domain name?
Name for location of resources on the Internet - unique to each site. Stored in DN server and each have a corresponding IP
What is a domain name server?
Phone book of Internet - translates URLs into IP address. Easy and quick to update. No meaningful characters to remember
Properties of circuit switching?
- Data split into packets
- Connection established via dedicated route
- No other traffic can use route till transmission has ended
- Packets of data all follow same route
- continuos flow of data
- Arrive in sent order
- Hacker can intercept data as route is known
Example of circuit switching?
Telephone calls
Properties of packet switching?
- Data broken into packets
- Packets travel multiple routes
- More efficient
- No dedicated route
- best path for packet is determined along the way by the router
- Takes longer to assemble
- Hackers can’t intercept
Describe each part of a data packet?
Header - Contains source and destination address, sequence number and protocol
Payload - Contains data
Trailer - Contains checksum and end of packet
What is a WAP? +/-?
- Allows wireless devices to connect to a wired network
- Uses WI-FI Bluetooth or related standards
- usually connects to routers
- Can rely data between wireless and wired devices
\+No physical setup \+Quick to add new nodes \+Flexible deployment of nodes \+Allows visitor access -Lower performance
Advantages of private network?
- Control over security
- control over who has access to what
- Control over what software is provided
- Confidence of availability
What methods are employed in order to minimise risks of private networks?
- Redundancy = essential equipment is duplicated
- Sensible backup regime = there is always a copy of essential data stored somewhere else
- failover systems = detect abnormalities and automatically transfer operations to an alternative system
- Disaster recovery plan =necessary so that in the event of a major failure, procedures are in place to limit the impact of the failure and remedies are applied effectively
What is network layering?
- Problems can be broken into components each of which is easier to solve than the whole
- Example of divide and conquer
- different aspects of the networks functionality are conceptualised and developed separately
- Each layer concentrates on itself and only communicates with layers adjacent to it
- Are an abstraction: solve problems by identifying common patterns in real situations
What are the levels of domains?
- Top level = .uk, .edu, .fr, .com
- second level = .org, .co
- Third level = OCR, BBC, hodder
What is a NIC?
- network interface controllers. Work on physical layer
- Circuits that were plugged into a computers bus to produce signals that are placed on the transmission medium and also receive signals from it
- designed to work with particular network standards (most commonly Ethernet)
- Most computers now built with Ethernet circuitry built into their motherboards rather than requiring cards as ads-on
What is a MAC address?
- 48 bit ID allocated to a network device by the manufacturer
- quoted in readable groups of six bytes (octets as there are 8 bits in each byte) and then converted in hexadecimal (eg; 08:01:27:0E:25:B8).
- The first 3 octets ID the manufacturer of the equipment. The others are allocated in a way decided on by the maker too ensure each address is unique
What is a router?
- Device that connects networks
- receives data packets from one network and forwards them to another network based on the address information in the packet
- work out where to send a packet according to either a table of info about neighbouring networks or by using an algorithm to determine the optimum next step for the packet.
- each router knows its closest neighbours and can therefore work out the optimum route.
How do routers differ between needs?
- home use = small routers that connect the users computer to the ISP
- business use = powerful high speed routers which are able to direct traffic according to the needs of the moment
What is a wireless access point?
- allow temporary connection of devices to a network
- BOYD common in business, university and public WI-FI
- allow connection of up to 100 meters. Allows signals to be intercepted
How does a WAP protect its ID?
-Hiding the SSID: A broadcast signal that ID’s a WAP, useful when network is to be used by outsiders
How does a WAP protect its signals?
-Encryption: WEP (Wired equivalent privacy) uses a static key of 40/60 bits to encrypt data. Drawback is all devices have to know the key. WPA and WPA2 have replaced this. They involve once-only cryptographic keys
What is the other way WAP protect themselves?
-limiting access: only allowing a certain list of MAC addresses to connect. Not practical when many new and unknown devices are likely to be connected
What is authentication? +/-
Users ID themselves with a user ID and confirm they are who they say they are with a password.
-easy to obtain a users password
-can be brute forced
+Most corporate networks require additional security such as a security device
+Captchas used to prevent automated attempts to gain access
What are Firewalls?
- Hardware, software (or a combination of the two) that control the traffic into and out of a network
- can be set up as a series of rules so that individual web addresses or specific computers can be blocked from accessing the network, or cannot be reached from within the network
- rules can be applied that cause messages containing certain words or other streams of bits to be filtered out
What is Packet filtering?
- Used in firewalls
- examines data packets as they pass the firewall and can reject them if they match a preset pattern.
- Operates at the lowest 3 levels of the OSI model
- other methods retain packets until it is established whether they are part of an existing message of the start of a new connection (check in revision textbook if this is right)
What are proxies?
- Proxy servers can act as firewalls. Computers interposed between a network and a remote resource
- if a user on the network requests a resource such as a web page, the request is picked up by the proxy server. This then either passes on the request to the destination or does not if the resource is on a banned list.
- The response is passed back to the proxy server, which may or may not forward it to the user.
- Never any direct contact between the users computer and the remote resource
What is encryption?
- transformation of data in such a way that unauthorised people cannot make sense of it
- used in networks because of risk that data might be intercepted
- Secret key is used to transform the original data and an algorithm is applied to that key
- critical part of VPN’s because the infrastructure is shared with a number of users
What is special about the algorithm applied in encryption?
- The algorithm is called a ciphertext
- the receiving device needs to have access to that key to decrypt the ciphertext and restore the original plain text message
- large keys more successful than small ones (64 bit mainly used)
- keys often subdivided so that parts are used to produce successive stages of encryption
