Communication and Networks Flashcards
(170 cards)
1
Q
What is data communication?
A
Data communication involves sending and receiving data from one device or computer to another
2
Q
What are the two ways in which data can be sent?
A
- Serial Data transmission
- Parallel data transmission
3
Q
What is serial transmission?
A
Data can be sent via an interface, one bit at a time over a single wire
4
Q
What is Parallel transmission?
A
- Data is transmitted several bits at a time (simultaneously) using multiple wires
5
Q
Parallel Transmission is only reliable over short distances. Explain why
A
- When parallel transmission is used, each wire has slightly different properties.
- This means bits could travel at slightly different speeds over each of the wires, leading to a problem known as skew
- Skew can get worse over longer distances, which is why parallel transmission is only suitable for short distances
6
Q
What are the advantages of serial transmission over parallel transmission?
A
- Low Cost - Reduced size and complexity of connectors used in serial transmission results in lower cost
- No interference - Cross talk causes interference between parallel wires, leading to data corruption. This worsens with increasing signal frequency and communication link length
- Reliability - Serial transmission is reliable over long distances as there is no skew of data that can occur in parallel transmission
- High net data transfer - Serial transmission can transmit data at high frequencies without interference. This means there is a higher rate of data transfer, even though less data is transmitted per cycle
7
Q
Serial transmissions can allow data to be transmitted at a higher data transfer rate at higher frequencies. Explain why
A
When transmitting data at high frequencies via parallel transmission, crosstalk occurs, which causes interference in the data transmitted by each wire. This causes data to be corrupted.
However, this problem does not occur in serial transmission and so it is possible to transfer data efficiently at higher frequencies, resulting in more bits transmitted per second without interference and therefore a higher data transfer rate.
8
Q
What is bit rate?
A
Bit rate is the number of bits transmitted serially per second
9
Q
What is baud rate?
A
Baud rate is the rate at which the signal changes
9
Q
How is bit rate calculated?
A
Bit rate = Baud rate x number of bits per signal
10
Q
What is baseband mode?
A
Two voltage levels are used, one to represent ‘0’, and the other to represent ‘1’.
11
Q
In baseband mode, bit rate and baud rate can be the same. Explain why
A
bit rate = baud rate x number of bits per signal. Since 1 bit is used to store each signal, the bit rate would be baud rate x 1 which is equal to the baud rate.
12
Q
Explain how it is possible for the bit rate to be higher than the baud rate
A
The bit rate can be higher than the baud rate if the number of bits used to store each signal is greater than 1.
13
Q
In a particular communications system, 32 voltage levels are used. How many bits can be in a group that is encoded by a voltage level?
A
5 bits, as 2^53 = 2
14
Q
What is bandwidth?
A
Bandwidth is the range of frequencies that a transmission medium can carry, measured in bits per second or Hz. It is the rate at which a transmission medium can send or receive bits per second.
15
Q
Explain why a higher bandwidth results in a higher data transfer rate
A
A higher bandwidth supports higher frequencies, which means more bits can be transmitted per second. This leads to a higher data transfer rate.
16
Q
Why is internet upload speed different from download speed?
A
Most users spend more time downloading than uploading so internet providers design their systems to give priority to downloading.
17
Q
What is latency?
A
Latency is the time delay between the moment that an action starts and the moment where its effect is observed.
In the context of data transmission it is the time delay between the moment that transmission of the first packet of data of a communication starts and when it is received at its destination.
18
Q
What is parity? Explain the parity system and what a parity bit is?
A
The parity system is a system to verify the correctness of data that could be erroneously changed during transmission.
A parity bit is an additional bit used to verify the correctness of other bits during data transmission. The parity bit is set at the transmitting end and checked at the receiving end.
19
Q
What is odd parity?
A
The total number of 1s in every byte, including the parity bit, is an odd number
20
Q
What is even parity?
A
The total number of 1s in every byte, including the parity bit, is an even number
21
Q
What is synchronous data transmission?
A
Using synchronous transmission, data is transmitted at regular intervals timed by a shared clocking signal, allowing for a constant and reliable transmission for time sensitive data.
22
Q
What are some examples where synchronous transmission is used?
A
- Time sensitive data
- In the CPU, the clock emits a signal at regular intervals and transmissions along the data bus, address bus and control bus start on a clock signal
23
Q
What is asynchronous transmission? Briefly explain the principles of asynchronous transmission
A
- In asynchronous transmission, one byte at a time is sent, preceded by a start bit and followed by a stop bit.
- The start bit and stop bit must always be different
- The baud rates of the sender and receiver must be set up to be the same or the signal won’t be received correctly.
24
The stop bit in an asynchronous transmission can be arbitrarily long. Explain why?
The stop bit is a "stop period", which can be arbitrarily long. This allows the receiver time to identify the next start bit and gives the receiver time to process the data before the next value is transmitted.
25
What are the differences between Synchronous and Asynchronous transmission?
- Synchronous transmission needs a common clock signal, whereas in asynchronous transmission there is no clock sharing.
- Synchronous transmission has a more complex interface because of the additional clock signal, whereas asynchronous transmission is cheap and easy to implement.
- Synchronous transmission allows for faster transmission, whereas asynchronous transmission is slower as extra bits need to be transmitted
- All parallel transmission is synchronous e.g. CPU, video/audio streaming, whereas asynchronous transmission can be used when data is sent sporadically e.g. via a mouse or keyboard.
26
What is a protocol?
A protocol is a set of rules relating to communication between devices.
27
Why is a communications protocol necessary when communicating over a network?
A protocol allows equipment from different supplies to be networked
28
What standards can protocols cover?
- Physical connection
- Cabling
- Mode of transmission
- Data format
- Error detection and correction
29
What is a LAN?
A Local Area Network consistes of a number of computing devices such as PCs, printers, scanners and a central server, on a single site or a single building, connected together by cables.
30
What is a LAN used for?
A LAN allows user on the network to:
- Communicate with each other
- Share hardware devices such as printers and scanners
- Share data
31
What is a topology?
A topology is the layout of a network
32
What are the characteristics of a Physical Bus Topology
In a physical bus topology:
- All network devices are connected to a single backbone cable
- The ends of the cable are plugged into a terminator
33
What are the characteristics of a Physical Star Topology?
In a physical star topology:
- Each node is connected to a central node
- The central node may be a switch or a computer that acts as a router
- The switch keeps a record of the unique MAC address of each network device, which helps it to identify where to route messages
34
What is the difference between a Physical and Logical topology?
Physical topology refer to the actual architecture of a network, whereas a logical topology refers to flow of data packets on the network
35
What are some advantages and disadvantages of Bus Topology?
- Cost: Bus Topology is inexpensive to install as it requires less cabling and no additional hardware (Advantage)
- Fault isolation: If the main cable fails, network data can no longer be transferred to any of the nodes. This makes it harder to identify any faults in the network (Disadvantage)
- Performance: Performance degrades with heavy traffic and high transmission speeds (Disadvantage)
- Low security: All computers on the network can see all data transmissions (Disadvantage)
36
What are some advantages and disadvantages of Star Topology?
- Cost: Star topology is more expensive to install due to increase cable length and additional hardware i.e. central node (Disadvantage)
- Fault isolation: If one cable fails, only one node is affected so easy to isolate faults (Advantage)
- If the central node goes down, the whole network goes down (Disadvantage)
- Performance: Consistent performance, even when network is being heavily used and at high transmission speeds (Advantage)
- Security: Highly secure as messages are sent directly to the central node and cannot be intercepted by other nodes (Advantage)
- Easy to add devices to the network without disruption (Advantage)
- No problem with data collisions as each node has its own cable to the central node (Advantage)
37
What is a NIC?
A Network Interface Card provides a computer with a full time, dedicated connection to a network. Each card represents a device and can prepare, transmit and control the flow of data on the network. A computing device must have an NIC in order to connect to a network.
38
What is a MAC address?
A MAC address is a unique, 48 bit (12 hex digits) long address that is hardcoded into the NIC of a device during manufacture. It uniquely identifies a particular device so data packets can be routed to and from them.
39
How can a MAC address be used to track a device?
When you walk around with a device containing an NIC, the device will scan for nearby Wi-Fi networks and broadcasts its MAC address
The MAC address is then logged by the network.
40
What are implications of MAC addresses on personal privacy?
Devices scan for nearby Wi-Fi networks and broadcast their MAC address.
As the devices pass by enough networks, broadcasting their address each time, the information could be used to track your complete movements throughout the day.
This is an infringement on personal privacy.
41
How could advertisers use tracking information?
Advertisers could know the areas you visited and could try to target ads specifically towards you
42
Who else might use this information?
Security services and police can use this for:
- Investigation of criminal cases
- Keeping track of the movements of individuals they are interested in
43
What is MAC address filtering?
Networks use MAC address filtering to allow only devices with specific MAC addresses to connect to a network.
44
What is a Client- server network?
In a client- server network, one or more computers known as clients are connected to a powerful, central computer known as a server.
Each client may hold some of its own files and resources such as software and can also access resources by the server.
45
What are some examples of client-server networks?
File server: Holds and manages data for all the clients
Web server: Manages requests for clients to access the web
Print server: Manages print requests sent by clients
Database server: One or more database clients accessing a database server and running queries against them
46
What are some advantages of client-server networks?
Sharing: Data and expensive/powerful resources can be shared between a large number of clients
Security: Security is better as files/data are stored in a central location and access rights are managed centrally
Backup: Backups are done centrally, recovery procedures recover lost data
Data consistency: Data consistency is maintained as only one copy of the data is held. Data consistency means each client sees a consistent view of the data, including visible changes made by the clients own transactions and transactions of other clients/
47
What are some disadvantages of client-server networks?
Expensive: Client server networks are expensive to install and manage as the hardware involved is expensive (expensive server-grade hardware and software), and professional IT staff are needed to maintain the servers and run the network.
48
What is a peer-to-peer network?
In a peer to peer network there is no central server i.e. decentralised. Individual computers and connected together either in a LAN or WAN so they can share files
49
In a small LAN, such as a home or small office, a peer to peer netweork is a good choice because:
It is cheap to set up
Not difficult to maintain
Enables users to share resources such as a printer or router
Scalable
50
How do P2P networks offer scalability?
Each peer potentially acts as a server.
This avoids the bottleneck of most distributed systems by causing the number of servers to increase linearly with the number of clients
51
What are some downsides of peer-to-peer networks?
- Widely used for online piracy as it's impossible to track the files which are being illegally downloaded
- Security Risks
- Lack of centralised control
52
Where are client-server networks used?
- Schools
- Hospitals
- Small/medium large businesses
53
Where are peer to peer networks used?
- Home
- Small office
- Video on demand
54
Why is it illegal to download music/movies from some internet sites?
- Most songs and movies that appear on file-sharing sites are protected by copyrights e.g. "Copyrights, Designs and Patents Act 1988"
- It is illegal to download any movies or music that is copyright
55
What are the consequences of online piracy?
- Financial loss to songwriters, filmmakers
- The artists do not get royalties for their work
- This is more significant in the case of budding artists
-Filmmakers and music companies will have less funding for new works
- This has a wider impact on the industries as a whole and everyone who works in them
56
What is the internet?
The Internet is defined as:
○ a network of interconnected computer networks
○ which uses an end-to-end communication protocol
57
Explain the physical structure of the internet
Each continent uses backbone cables connected by trans-continental leased lines fed across the sea beds.
ISPs connect directly to this backbone and distribute the internet connection to small providers who in turn provide access to individual homes and businesses
58
What is a URL?
A Uniform Resource Locator is the full address for an internet resource. It specifies a resource on the internet, including its name and a file type, so that a browser can go and request it from a web server
59
Explain why internet registrars are needed
Internet registrars are needed to ensure that a particular domain name is only used by one organisation, and they hold records of all existing website names and the details of those domains that are currently available to purchase. All registrars must be accredited by their governing registry
60
What are internet registries?
Internet registries are five global organisations governed by ICANN, with worldwide databases that hold records of all the domain names currently issued to individuals and companies, and their details. They also allocate IP addresses and keep track of which addresses a domain name is associated with as part of DNS
61
What details are stored by internet registries?
- Registrant's name
- Type (company or individual)
- Registered mailing address
- The registrar that sold the domain name and date of registry
62
Explain the purpose of a domain name
A domain name identifies the area or domain that an Internet resource resides in. These are structured into a hierarchy of smaller domains and written as a string separated by full stops.
Each domain name corresponds to one or more IP addresses
63
Explain how DNS is used when a user requests a web page from its URL
The DNS catalogues all domain names and IP addresses in a series of global directories that domain name servers can access in order to find the correct IP address location for a resource.
When a webpage is requested using the URL a user enters, the browser requests the corresponding IP address from a local DNS. If that DNS does not have the correct IP address, the search is extended up the hierarchy to another larger DNS database. The IP address is located and a data request is sent by the user's computer to that location to find the web page data.
64
Why are IP addresses not used to access websites instead of alphanumeric addresses?
Domain names are much easier for humans to remember and type in than a string of numbers in an IP address.
65
What is a FQDN?
A Fully Qualified Domain name is one that includes the host server name, for example www. or ftp. or mail.
66
What is an IP address?
An Internet Protocol address is a unique address that is assigned to a network device. It indicates where a packet is to be sent to or has been sent from
67
What is Wi-Fi?
Wi-Fi is a local area wireless technology
It enables you to connect a device to a network resource or the internet via a wireless access point (WAP)
68
What are the components required for a computer to connect to WiFi?
In order for a computer device to connect to a wireless network, the computer needs a wireless network adaptor.
The combination of a computer and the wireless network adaptor is called a station.
It also needs a wireless access point to connect to. The WAP usually connects to a router, but can also be an integral part of the router itself.
69
How do stations work?
All stations share a single radio frequency communication channel, and each station is constantly tuned in on this frequency to pick up transmissions. Transmissions are received by all the stations within range of the wireless access point.
70
How do stations work?
All stations share a single radio frequency communication channel, and each station is constantly tuned in on this frequency to pick up transmissions. Transmissions are received by all the stations within range of the wireless access point.
71
How does a WiFi network work?
- A wireless router receives information from the internet,
- Translating it into a radio signal and sends it to the computer's network adapter
- The station then receives this transmission and translates it into data
72
How can a WiFi Network be secured?
- Using a WPA/WPA2 enabled wireless network adapter provides strong encryption of data transmissions, with a new 128-bit key generated for each packet sent
- SSID requires a user of a local network to enter in a password before they can join
- Disabling SSID broadcast allows only users that know the name of the network to join
- MAC address whitelisting allows only trusted devices on the networks
73
What is an SSID?
An SSID is a locally unique identifier for a wireless network
74
Why might SSID broadcast be disabled?
- Broadcasting SSID publicly announces your wireless network and can be seen as a security weakness.
- hence, disabling SSID broadcast will hide the network from others wanting to connect to it using the name
75
What is a data collision?
A data collision occurs as a result of simultaneous data packet transmission between two or more nodes. This can result in data corruption.
76
What is CSMA/CA
CSMA/CA stands for Carrier Sense Multiple Access with Collision Avoidance.
It is a protocol for carrier transmission in wireless local area networks that attempts to avoid collisions occurring on a data channel
77
What is the "hidden nodes" problem?
- The hidden nodes problem occurs in the case of wireless transmissions.
- The problem occurs when two nodes, unaware of each other's existence, transmit simultaneously to an access point
- This results in interference at the access point, which causes no data to get through
78
What is CSMA/CA with RTS/CTS?
- In the CSMA/CA protocol, prior to transmitting data, a node first listens for signals on the network to determine whether another node is transmitting.
If a signal is detected, it waits for a random period of time for the node to stop transmitting and listens again.
With RTS:
- Having determined that no other node is transmitting, the node wanting to transmit sends a Request To Send signal
- If and when the channel is idle, the WAP responds with a "Clear To Send" signal which allows the node to transmit data
79
What is packet switching and how does it work?
- Packet switching is a method of communicating data packets across a network on which other similar communications simultaneously occur.
- Communications are shared across many cables to allow efficient use of them
80
What is an example of packet switching?
- Website data you receive arrives as a series of packets
- An email leaves you in a series of data packets
81
What are data packets? What are their characteristics?
- Data that is to be transmitted across the internet is broken down into manageable chunks known as packets.
- The size of each packet can be fixed or variable and ranges between 500-1000 bytes
- The packet contains 3 sections:
- Header
- Payload
- Trailer
Packets can be sent from sender to recipient along entirely separate routes and can arrive in a different order
82
What is a checksum?
- A checksum is a small sized block of data that is calculated from the payload data of a data packet
- It is used to check for data integrity and transmission errors
- Can also be referred to as a Cyclical Redundancy Check (CRC)
83
How is a CRC used?
- The CRC is calculated prior to transmission and is added to the trailer of the packet
- It is recalculated upon receipt for each packet using the payload data
- If the CRC totals prior to transmission and upon receipt differ, the packet is refused with suspected data corruption and a new copy is requested from the sender
84
What information is included in the packet header to reassemble packets in the correct order?
The header contains:
- Sender IP address
- Recipient IP address
- Protocol being used with the packet
- Packet sequence number e.g. 2/5
- Hop Limit/Time to live (TTL) - after which the packet expires and is discarded
85
Why is the sender's IP address included in the packet header?
The sender's IP address is included to:
- Identify the sender
- To send a request back to the sender for a duplicate copy if it includes data with transmission errors
- Notify the sender if a packet cannot be delivered
- Aid with firewalls' packet filtering
86
What is a router?
Routers are used to connect at least two networks, commonly two LANs or WANs, or to connect a LAN and its ISP's network
87
What is a hop?
The act of traversing between one router and the next is referred to as a hop
88
Why is the recipient's IP address included in the packet header?
The recipient's IP address is read by the router in order to forward the packet to the recipient in the fastest and least congested path to the next router/
89
Explain how routers identify the most efficient path for data transmission
- Routers use routing tables to store and update the locations of other network devices
- A routing algorithm e.g. Djikstras is used to find the most updated route.
The most optimum route is that which is the least congested i.e. has the least network traffic or takes the least number of hops
90
Why should IP addresses be unique?
So that data is transmitted towards the intended recipient
91
Explain the differences between a router and a gateway
A router operates between two networks using the same protocols
A gateway operates between two networks using different protocols
92
Explain how a gateway routes packets between networks using different protocols
The header data of the packet is stripped between networks using different packets
New header data of the format of the new network is added to the packet
The packet is then sent by the gateway towards its destination
93
What is a firewall?
A firewall is a security checkpoint designed to prevent unauthorised access to a between two networks:
- A trusted, internal network
- An untrusted, external network, often the internet
94
Explain how firewalls prevent unauthorised access between two networks
- A typical firewall consists of a separate computer containing two NICs
- One NIC is connected to the internal network and the other is connected to the external network
- Using special firewall software, every data packet that attempts to pass between the two NICs is analysed using pre-configured rules, then accepted or rejected
95
What is packet filtering/static filtering?
- Packet filtering/static filtering controls network access according to network administrator rules and policies by examining:
- source/destination IP addresses in the packet header
- Protocols being used
- Port numbers that are trying to be accessed
- If the IP addresses match those on the admin's 'permitted' list, they are accepted. Otherwise the packet is blocked
96
What is the difference between packet rejection and packet dropping?
- When a packet is rejected by a firewall using packet filtering, it can do so by dropping or rejecting the packet
- A dropped packet is quietly removed, whereas a rejected packet will cause a rejection notice to be sent back to the sender
97
What is stateful inspection? How is it different to static inspection?
Rather than relying on the IP addresses, protocols used and port numbers, stateful inspection examines the payload of the data packet to better assess it for security through making dynamic, contextual rules based on the data.
98
Explain how stateful inspection provides greater security than simple packet filtering
Stateful inspection provides greater security by:
- Payload inspection: Examines payload data of packet rather than just header
- Port scanning: It can also create temporary contextual rules to keep open the ports that are currently being used
99
What is a proxy server?
A proxy server intercepts all packets entering and leaving a network, hiding the true network addresses of the source from the recipient
This enables privacy and anonymous surfing
100
What are the beneficial features of proxy servers?
- Privacy: Proxy servers hide the true network addresses of the source from the recipient, enabling privacy and anonymous surfing.
- Caching: Proxy servers maintain a cache of websites commonly visited, speeding up user access to web page data and reducing web traffic
- Request filtering: Proxy servers are often used to filter requests providing administrative control over the content that users demand
101
Explain how proxy server caching works
- A proxy server maintains a cache of commonly visited websites
- When a web page is requested, the proxy server returns it from the cache if present
- If the page is not present in the cache, the proxy server makes a request of its own and returns the data to the user, adding the web page to its cache
102
What is encryption? What are two main components required
- Encryption is the process of scrambling data so that it becomes very difficult to unscramble and interpret
- The process of encryption is carried out using a cryptographic algorithm and a key
103
What is symmetric encryption/private key encryption
A type of encryption where the same key is used to encrypt and decrypt the data
104
What are the risks associated with using symmetric encryption?
- Key loss: If either party loses the key or the key is stolen, the system is broken
- Key exchange problem: If the key is intercepted along with its data, the message can be deciphered
105
How does asymmetric key encryption work?
- Asymmetric encryption uses two separate, but related keys i.e. public key and private key
- The public key is made public so that others wishing to send data can use it to encrypt the data
- The public key cannot decrypt data
- The private key is only known by the recipient and is used to decrypt the data
- It isn't possible for the private key to be deduced from the public key
106
What are the risks of asymmetric key encryption? how can they be resolved?
It is possible that a message encrypted by the recipient's public key is sent by a malicious third party user impersonating a trusted user
To prevent this, a message can be digitally signed to authenticate the user
107
How does a digital signature work?
- A hashtotal/checksum/digest of the unencrypted message is calculated
- The sender encrypts the digest using their private key
- The encrypted digest becomes the digital signature
- The signature is attached to the message to be sent
- The whole message, including the signature is encrypted using the recipient's public key
- The recipient then:
- Decrypts the message using their private key
- Decrypts the signature using the sender's public key
108
Why are digital signatures encrypted using a private key and decrypted using a public key i.e. why is it the opposite of asymmetric encryption?
- In the case of asymmetric encryption, it is important to restrict who is able to decrypt the message, so the private key is used for decryption
- However, in the case of digital signatures, it is important to restrict who signs the digital signature, so the private key is used for encryption
109
What are hoax digital signatures and how do we mitigate against them?
- Hoax digital signatures could be created using a bogus private key claiming to be that of a trusted individual
- In order to mitigate against this, a digital certificate verifies that a sender's public key is formally registered to that particular sender
110
Digital signatures vs Digital certificates
Digital signatures verify the trustworthiness of message content
Digital certificates verify the trustworthiness of the message sender
111
What does a digital certificate contain?
- The certificate's serial number
- The name of the holder
- The certificate's expiry date
- The public key of the holder
- the digital signature of the CA (certificate authority)
112
What is malware?
- Malware, or malicious software, is a type of software that is designed to damage or destroy data and/or systems
113
What are viruses?
- Viruses are a type of malware that attach themselves, or conceals themselves within a host file. Viruses require a host file through which they can replicate and cause damage
114
What are worms?
- Worms are standalone software that do not require a host file to run. They are self-replicating and can exploit network vulnerabilities in order to spread to other devices, or through networked applications such as email
115
What are Trojans?
Trojans are a type of malware that appear trustworthy and legitimate but are actually malicious software, usually spread through phishing
116
What are system vulnerabilities?
System vulnerabilities arise from human error or software bugs
- Human errors:
Switching off firewall
Failure to renew virus protection
Unrestricted admin rights to key file ares
- Software errors:
Cracks in software where data is passed from one function, module or application to another
117
How can we protect against viral threats?
- Improve code quality in order to prevent issues such as: SQL injection, buffer overflow
- Social engineering education: Awareness about phishing, spam filters
- OS and antivirus updates
118
What is a TCP/ICP protocol stack? What are the four layers of the stack?
The TCP/IP (Transmission Control Protocol/internet protocol) stack is a protocol stack.
It is a set of networking protocols that work together as four connected laters, passing incoming and outgoing data up and down the layers during network communication.
The four layers are Application, Transport, Network, Link
119
What is a short summary of the TCP/IP stack?
- Various protocols operate at each end of the stack
- In each layer, data is encapsulated within the packet
- At the receiving end, the packet is unwrapped
120
Describe the role of the Application layer, when sending packets and receiving packets.
- Sits at the top of the stack
- The role of the application layer is to interact with the user via application software e.g. browser, email client, FTP client
- Application layer selects appropriate protocol for the communication (e.g. HTTP, POP3, FTP)
At the receiving end, it receives the packets from the transport layer and presents the data to the user via application software
121
Describe the role of the transport layer when sending and receiving packets
When sending packets:
- Transport layer uses TCP
- It establishes an end-to-end communication with the recipient computer
- The data is split into packets and labelled with the packet number, total number of packets, port number, sequence number etc
- Transport layer also deals with the acknowledgment when transmission is successful and requests transmission of lost or erroneous packets
When receiving packets:
- Uses the port number to determine the application to pass data to in the Application Layer
- Removes port numbers and splits packets into correct order
122
What is the role of the network layer when sending and receiving packets?
When sending packets:
- The network layer is involved with packet routing
- The network layer adds the source and destination IP addresses
- The combination of IP address and a port creates a socket
When receiving packets:
- The network layer strips IP addresses and passes the packets to the transport layer
123
What is the role of the link layer when sending and receiving packets?
The link layer is the physical connection between network nodes
When sending packets:
- It adds the unique MAC addresses identifying the Network Interface cards of the source and destination devices
When receiving packets
- The link layer strips the MAC addresses and passes them to the network layer
124
How does the addition of the MAC address in the link layer help routing?
At the link layer, the MAC addresses identifying the NICSs of the devices is added on top of the IP addresses of the devices.
This helps with packet routing to the correct destination as the correct network can be identified using the IP address and the correct destination device within this network is identified by the MAC address
125
What is a port?
A port determines what applications may deal with a data packet as it enters your computer/application layer during data transmission.
Several common application level protocols may use standard ports on the server
126
What is the port number for FTP data?
20
127
What is the port number for FTP control instruction?
21
128
What is the port number for SSH remote login?
22
129
What is the port number for Telnet (unencrypted remote login)
23
130
What is the port number for SMTP
25
131
What is the port number for HTTP
80
132
What is the port number for HTTPS
443
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What is the port number for POP3
110
134
What is the port number for IMAP
143
135
What is HTTP and HTTPS?
HTTP is a protocol that is used to load webpages using hypertext links.
HTTP is a version of HTTP which is encrypted and secured using digital certificates. It is used to secure online transactions such as banking or shopping
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What is FTP? Explain what it involves
FTP (File Transfer Protocol) is a protocol used to transfer files over a network
FTP clients sit on top of the FTP and present the user with a file management screen, showing the file and folder structure of the local and remote server
User actions such as dragging and dropping are converted into FTP commands
Can be used anonymously or logged in
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What is SSH? What is it used for
SSH (Secure Shell) is used for remotely accessing and managing a computer
It is a modern and secure replacement of Telnet
It is used by network admins to remotely manage their business servers
It uses public key encryption, requiring a digital certificate to authenticate the user
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What is SSH tunnelling and where is it used?
An SSH tunnel consists of an encrypted tunnel created through an SSH connection
Users may set up SSH tunnels up to send unencrypted traffic through an encrypted channel
The data traffic is set up to go inside the tunnel so that it cannot be intercepted or eavesdropped
Application-level commands can be used within the tunnel
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What is the role of email servers?
Email servers are used for sending and receiving emails emails.
SMTP is used to send emails
POP3 is used for receiving emails,POP3
communicates with email servers to check for
and download any new messages.
When an email is opened, it is deleted from the server so there could be an issue with synchronicity when using multiple devices
140
What is the role of a web server?
A web server is used to host websites. Web pages are stored in the form of text files written in HTML, CSS and/or Java Script.
A web server handles client requests using HTTP. The web server responds to these requests by delivering the website content to the user
The web server uses load balancing between several servers to manage high traffic to and from the site and deliver a smooth experience to users.
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How is a web page rendered by a browser?
When a browser receives an HTTP response from a web server, it parses the document containing the HTML, CSS and JS code into a hierarchal model.
The HTML is broken down into a hierarchy of tags called the DOM (Document Object Model) tree inorder for the browser to structure the code. The CSS styling form their own CSSOM and are related to the corresponding HTML tags. The JS is parsed and executed. Further HTTP requests are made to download any other resources eg. images.
The browser then renders the image on the screen.
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What is an IP address? What are the two forms of IP addresses?
An IP address is a unique, numerical address used to identify computers or network nodes trying to communicate over IP on the internet.
The two standards of IP addressing are IPv4 and IPv6
143
What does an IPv4 address look like
32 bit number written in dotted decimal notation
Each part represents an 8-bit binary pattern giving a range of 0-255 for each number
144
What is the structure of an IPv6 address?
Consists of a 128 bit number represented as a hexadecimal string
Offers a larger range of addresses than IPv4
Increasingly becoming the new standard of IP addresses
145
What are some reserved addresses
x.x.x.0 - network identifier
x.x.x.255 - broadcast address on the subnet
146
What are reserved IP addresses?
Reserved IP addresses are specific ranges of IP addresses that have been set aside for specific purposes
These reserved IP addresses are not available for general use.
They are typically used by organisations or service providers for internal networks or special purpose networks.
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What is the structure of an IPv4 address?
An IPv4 address is split into Network ID and Host ID. The network ID uses the first bits in the address
148
What is addressing? What are two types of addressing?
Addressing is a means to define the proportion of the network and host IDs within an IPv4 address
The two types are:
- Classful Addressing
- Classless Addressing
149
What is Classful addressing? Give examples of some classes
Classful addressing is an addressing system that uses a system of classes to define the proportion of network and host IDs within an Ipv4 address.
Class A:
- Very few network identifiers (7 bits for network id, so can join max 126 networks)
- Millions of host addresses
- Suitable for large organisations
Class C:
- Millions of networks (24 network id, 8 bit host)
- Few hosts
- Suitable for small organisations
150
What are the limitations of classful addressing?
- Classful addressing does not offer flexibility as the division between network and host IDs always happens in a small number of fixed positions
151
What is classless addressing? Give an example
The classless addressing system specifies the number of bits used for the network ID (subnet mask)
- In 103.27.104.92/24 the '24' suggests that the first 24 bits are reserved for the network ID and the rest reserved for the host ID
152
What is a subnet mask?
A subnet mask is a 32 bit number created by setting the network bits to 1 and host bits to 0
It identifies the number of bits reserved for the network ID
153
How can the network ID be identified from an IP address? Why is this useful?
By ANDing with a subnet mask. This is helpful in identifying whether the source and destination IP addresses are on the same subnetwork.
154
27 bits have been allocated to
the network/subnet identifier.
In binary, write out the subnet mask that has been programmed into the devices on the network.
11111111111111111111111111100000
155
What is the maximum number of devices that could be connected to a subnet with host ID of 5?
2^5 - 2 = 30
156
What is a subnet?
Subnets are network segments within a large IP network
157
What are the advantages of subnets?
- Ease of management for network admins
- Routing efficiency
- Improved security
- Improved speed
- Improved reliability
- Prevention of data collisions during transmissions
158
How is a subnet fragmented? (classful addressing)
A subnet ID is created by using the most significant bits from the host ID section of the IP addresses
159
Differences between public and private IPs
- Public IPs are globally unique whereas Private IPs only have to be locally unique
- A public IP can be directly connected to from an external network, whereas a private IP requires a gateway or router to connect through
- Public IPs are allocated by a central internet registry, whereas private IPs are allocated by a network admin
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How does a home router establish a connection between home devices and the internet?
The home router has a public IP to communicate with other devices on the internet and be routed to, and has a private IP to connect to the same internal subnet as your computers would on your home LAN
161
Describe:
* the purpose of the DHCP system
* why the DHCP system is used
* what will happen when a device uses DHCP to connect to a public network
* DHCP is used to automatically assign a dynamic IP address from a pool of available addresses to a computer attempting to operate on a public network such as a hotspot.
* It facilitates efficient use of a limited pool of IP addresses by enabling active computers to request an IP address for the duration they are online and then release the address back to the pool when not in use.
1. Host sends request to discover a DHCP server
2. DHCP server offers IP address to host
3. Host accepts offer of configuration from a DHCP server (by echoing the accepted
configuration back to the server)
4. DHCP server confirms that configuration has been allocated to host
162
What are the advantages of DHCP?
* IP configuration information is automatically setup for the client machine by the DHCP server
* A mobile device moving from one subnet to another will be automatically rediscovered by the DHCP server as soon as it broadcasts a new discover signal
* Less likelihood of an IP address conflict
163
What is NAT and why is it needed?
* Private IP addresses are non-routable and non unique
*Hence, they can't be used for routing packets on the internet
* To allow external access to a privately addressed device, Network Address Translation
* NAT converts IP addresses as they pass between a public address space (e.g. router) and a private address space (e.g. LAN)
164
How does NAT work?
* An outgoing server request made by a computer on a private network contains its own IP address & port number
* Router logs this as an entry in a translation table
* The router swaps the packet IP address and port number with its own external IP address and port number
* An incoming response is then rebadged with the original internap IP address and port number from the translation table
165
What the benefits of NAT?
* NAT allows external access to a privately addressed device
* NAT provides a solution to the lack of public addresses in IPv4, enabling everyone to have a unique address
*Provides an additional layer of security by automatically creating a firewall between internal and external networks by only allowing in network traffic if a device on the private network requested it.
* The firewall protects the identity of the network and doesn't show internal IP addresses to the internet.
166
Why is port forwarding needed and how is NAT used for port forwarding?
Since there is no direct connection to the server, the NAT needs to forward all incoming requests to a socket (particular IP address and port) to a port of the internal server
167
AN FTP server inside a company network contains files that employees can access outside of the office network. Explain how port forwarding can be used to access internal files.
Port forwarding is required because an FTP client operating in a public address space cannot directly access an FTP server hosting files on a private network.
The NAT forwards inbound data packets using a particular IP address and port to a port that the private host within the LAN is using
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Why is port forwarding needed and how does it work
Port forwarding is used when a client needs to communicate with a server that is connected to a private network.
The client sends packets to the public IP
address of the router belonging to the
server’s private network.
The packets sent by the client contain the port number of the application running on the server that the client wishes to access.
The private network’s router then forwards the packets to the server using NAT.
169
How can the authenticity of a message sent using a digital signature be checked?
The recipient will first decrypt the message using their private key.
They can then decrypt the digest using their public key.
They can use the message to recalculate the digest, and compare this recalculated digest with the original digest.
If the two digests match, the recipient can be sure that the message is authentic.
