Networks and internet Flashcards
[PPQ] How are checksums used during communication?
The checksum is calculated from the data in the packet before it is transmitted, and is sent along with the packet. The checksum is recalculated at the receiving end, and if the two checksums do not match then the data has been corrupted
[PPQ] Why can parallel communication not be used at long ranges?
High risk that data skew will occur, and there could be interference between the multiple wires
What are the advantages of ring topologies?
Using a ring topology reduces the risk of packet collisions. The speed of data transfer also decreases less under heavy load
What are the advantages of bus topologies?
Cheaper, easier to install
What is a protocol?
A set of rules that govern how devices or programs should communicate
[PPQ] Explain the differences between the ways in which parallel and serial communication
is carried out.
Serial sends one bit at a time whereas
parallel sends multiple bits simultaneously. Serial uses a single wire whereas parallel uses several.
[PPQ] Why do peripherals use serial communication?
Parallel communication can only be used over short distances because of data skew (where bits travel at different speeds and arrive at different speeds)
[PPQ] What is baud rate?
Number of signal changes per second
What is a packet?
A container in which data is transmitted over networks. It contains the addresses of the recipient and the sender
What is a packet switched network?
A network in which data is set in packets. One message is split into multiple packets, each of which is sent to the recipient by the best possible route, before being reassembled with other packets
What happens to a packet as it goes through a network?
It passes through a number of routers before reaching its destination. A router uses the recipient address on a packet to determine where to send the packet. Every time that a packet passes through a router, a hop is said to occur.
What is a packet’s “time to live”?
The number of hops it can do before it’s deleted by the router
What are the primary components of a packet?
Sender’s address, receiver’s address, packet contents, time to live, sequence number
What are gateways?
Network devices that allow two networks to communicate using different protocols. It modifies packets by stripping away everything but the contents, and giving them new metadata which complies with the new protocol
What is the meaning of the “https://” part in a URL?
The protocol being used to acceess the file
What is the meaning of “www” in a URL?
Subdomain
What is a FQDN (Fully Qualified Domain Name)?
A domain name that specifies an exact resource and can only be interpreted in one way
What are IP addresses?
They are addresses that are assigned to every device connected to the internet. Since they are not user-friendly, easily-recognisable domain names map to IP addresses
What is a DNS (domain name server)?
A server that translates domain names into their corresponding IP addresses. DNSs contain large tables mapping domain names to IP addresses. If the website the user is trying to access is obscure or rarely visited, the DNS might not have it in its records, and will have to pass on the request to another DNS, meaning small websites sometimes take longer to load
What is an internet registry?
An organisation responsible for the allocation of IP addresses. They protect the depleting pool of unallocated IP addresses, When a new IP address is requested, an internet registry will first
look for a previously allocated IP address that has become unused rather than allocate a brand new IP address straight away.
What is a firewall?
Software or hardware that monitors incoming internet traffic and blocks it if it deems it to be malicious, based on its source IP address or the protocol it’s using
What is stateful inspection?
Examining the contents of a packet before deciding whether to allow it through the firewall
What is a proxy server?
A server that sits between a public network and a private network is called a proxy server. These devices manage every packet that passes between the two networks. Firewalls can be said to act as proxy servers. When a device in a private network sends a packet through a firewall and into a public network, the packet’s “sender” address is that of the firewall, rather than the device’s
private IP address. This provides some degree of anonymity.
What is a digest?
The value of the digest depends on the content of the message and will not be the same if the message is changed
How do digital signatures work [PPQ]?
- A digest of the message is created
- This digest is encrypted with the sender’s private key and appended to the message
- The message and appended digest are encrypted
with the recipient’s public key, meaning that only the
recipient can decrypt the information. - When the recipient receives the message, they first
decrypt it using their private key - The recipient creates a new digest from the message
- The digest is decrypted using the sender’s public key, to produce the original digest
- If the two digests match, the receiver knows the message was sent by the sender
What is a digital certificate?
A digital certificate verifies ownership of a key pair used in asymmetric encryption and can be used to check that a fake key pair isn’t being used by an impostor. Issued by certificate authorities, these files contain: a serial number, the owner’s name, an expiry date, the owner’s public key and the certificate authority’s digital signature.
What is CSMA/CA?
A protocol that allows multiple devices to transmit data at high speeds without interfering with each other
How does CSMA/CA with RTS/CTS work?
Computer with data to send monitors/listens for data signal
If another transmission in progress then continue to wait
When no signal present computer sends a Request to Send
Two computers could start transmitting simultaneously if they both detect there is no data signal
Teceiver/WAP responds (to RTS) with a CTS signal, which blocks any other transmissions from nodes in range
When CTS received then start to transmit. If CTS not received continue to wait (until transmission ends)
After transmitting the transmitter waits to receive acknowledgement packet (to confirm data received and not corrupted, notifies other computers that they can transmit again)
If no acknowledgement received, wait a random time period and listen again / retransmit.
What is a SSID?
A locally unique 32-character code that identifies a device on a wireless network
What is a MAC address?
A 48-bit address hard-coded into the NIC that uniquely identifies each device on a network
What is a port?
A code used to identify a specific application on a network
What is TCP/IP?
A set of protocols that govern transmission of data packets. IP controls the delivery of the packets and TCP reassembles them on arrival.
What is the application layer of TCP/IP?
Handles DNS, HTTP, SMTP, FTP, converts incoming and outgoing data from one format to another. FTP client software can run on devices, allowingthem to connect to FTP servers which sendfiles requested by the client.Clients can access FTP servers anonymouslyor non-anonymously by logging in with ausername and password.
What is the transport layer of TCP/IP?
Checks that all packets have arrived and there are no errors in the packets. It also decides which path data will take between nodes. It passes packets to the process that needs them by reading the port number.
[PPQ] What is the network layer of TCP/IP?
Adds source and destination IP addresses to packets
Uses the destination IP address to determine which router to send the packet to next
Carries out error detection on the packet header
Splits data into datagrams
What is the link layer of TCP/IP?
Synchronises devices so the receiving device can manage the flow of data being received. It identifies which network topology is being used. It handles the CSMA/CA protocol
What is a socket?
Combination of an IP address and a port number. A socket address identifies which of the applications on the recipient device a packetshould be sent to.
What is HTTP?
The protocol that governs how multimedia files are transmitted around the internet. It handles the transmission of hypertext. Web servers hold web pages in text form, which they can deliver to clients using HTTP. Web browsers on the client receives these text files and renders them as webpages. HTTPS is an extension of HTTP that uses the SSL protocol for encryption.
What is FTP?
The protocol that controls the movement of files around the internet. It is used when downloading files from a website or uploading web pages to a server
What is SSH?
A protocol that improves the security of remote desktop connections. It does this by providing encryption, and providing a secure network of nodes for ccommunication. SSH commands are usually input through the command line
What are non-routable IP addresses?
Those that are used by devices on a private network. They are unique within the local network but are not allocated on a global basis. They connect to the internet through routers or proxy servers, which do have globally unique IP addresses
What is NAT (network address translation)?
The system that matches private IP addresses with public ones. This means that only one device on a LAN must have a globally-unique IP, the router. There is also increased security because the private IP address is not being broadcast over the internet
Explain how NAT works
The workstation on the LAN sends a packet to the server on the internet, requesting some data. It includes its internal IP address and port number.
The router replaces the IP address with its own public IP address, and a randomly generated port number. The router saves the original IP address and port to a table
When the server sends packets back, the router fetches the original IP address from the table and the process is done in reverse.
What are the advantages of JSON?
Easy for humans to understand and implement, easier for computers to interpret because each object and value is stored on its own line
What are the advantages of XML?
More freedom over what data types are created/used, more flexible
In which layer of the TCP/IP stack does a router operate?
Network layer
What is port forwarding?
A method used when a server inside a private network, with a private IP address, is used to provide services on the internet. The method is where the router is programmed so that all packets sent to it with a particular port number are forwarded to the web server
How does CSMA/CA with CTS work?
If the transmission medium is free, the data is sent. When no signal is present, the sender sends a RTS. The receiver responds to the RTS with a CTS. The RTS/CTS signal blocks any other transmissions from nodes in range. When CTS is received, the sender transmits. If CTS is not received, the sender will continue to wait. The receiver sends acknowledgement after all the data is received. If acknowledgement is not received, the sender waits a random amount of time and sends again.
What is a client port?
An unused port
Describe thin-client computing
A thin-client network is where the individual computers connecting to the network are very basic, and most of the processing is done on the more powerful server computers. The server contains the operating system and programs used by the client computers, meaning the client computers are just acting as terminals.
The client computers will be cheap to manufacture, since they have no secondary storage, little RAM, and a slower CPU. The server computers will need to have fast CPUs with many cores and lots of RAM and storage space in order to handle requests from all the clients. For the same reason, the network will also need to be high-bandwidth.
Explain how the TCP/IP stack in each host supports an e-mail client to e-mail server request at the same time as a web browser to web server request
The data passes between the different TCP/IP layers on the way to and from the server.
Client to server: The application layer chooses which protocol to use to communicate with the server (HTTPS in the case of the web server, POP3 in the case of the email server). The transport layer then opens communication with the server and splits the request into packets. It also decides what path the packets will take between the routers. The network layer then adds information to the packets such as source/destination IP address, and the link layer deals with transmitting the data physically from the client to the server.
Server to client: The server processes the client’s request and uses the IP address in the packets to determine where to send its response. It also uses the port numbers in the packets to determine which client-side process to send the response to.
What is a subnet?
A smaller subdivision of a network that makes routing data more efficient
What is subnet masking?
The process of using an AND operation on a subnet mask (usually 11111111.11111111.11111111.00000000) to verify that a device is in the same subnet
What is DHCP?
A protocol for allocating locally-unique IP addresses to devices as they connect to a network
Static/dynamic IP addresses
A static IP address never changes, a dynamic IP address is allocated every time a device connects to a network
What is SMTP used for?
To send/receive emails (to/from another email
server/client)
What is POP3 used for?
Managing and fetching delivered email messages from your mail server
[PPQ] What is the difference between peer-to-peer and client-server networking?
In client-server networking, resources are stored on the server, which provides services to the clients. It’s more centralised, secure and easy to back up but requires more expertise to set up. In peer-to-peer networking, resources are stored on each individual computer, and a computer can access resources from any other. All computers have equal status and can share devices such as printers. Managing security is more difficult but it’s cheaper to set up
[PPQ] Explain the difference between a physical topology and a logical topology
Physical topology is the arrangement of the connections between computers, while logical topology is the way data flows around the network
[PPQ] Differences between synchronous and asynchronous communication
Synchronous transmission: Data transferred at regular intervals, synchronised by a clock signal, useful for reliable, real-time, data such as VoIP video. Often used by parallel communication
Asynchronous transmission: Each byte sent as soon as it’s ready, parity bits and stop bits used to time and synchronise the data characters being transmitted
How are wireless networks secured?
Strong encryption of transmitted data using WPA (Wifi Protected Access)/WPA2, SSID (Service Set Identifier) broadcast disabled,
MAC (Media Access Control) address white list.
What does even parity mean?
The number of bits will always be even
How does an analogue to digital converter work?
Takes samples of the analogue signal at regular intervals, and the amplitude of each sample is approximated to an integer value. Each sample is then stored as binary data.
[PPQ] Explain why DHCP is used, and its purpose
To allocate IP addresses to computers connecting to a network. It reduces the time required to configure hosts, and allows IP addresses to be reused
[PPQ] What happens during a DHCP communication?
- Host sends request to discover a (DHCP) server
- (DHCP) server(s) offer configuration to host
- Host accepts offer of configuration from (a DHCP) server (by echoing the accepted configuration back to the server)
- (DHCP) server confirms that configuration has been allocated to host
[PPQ] Advantage of star topology over bus topology
Improved security as data is not sent to all nodes, speed not affected by number of connections/collisions. Network does not go offline if one PC malfunctions. No problem with collisions
Advantages of serial communication
Serial links do not suffer from data skew at long ranges, so are more reliable at these ranges
Cheaper to set up
There can be crosstalk between parallel lines at high frequencies
Higher frequencies can be used (due to no crosstalk) increasing data flow rate
What is bandwidth?
Range of frequencies a transmission medium can carry. The larger the range, the more bits can be transferred per second
What happens during asynchronous transmission?
Byte
CSMA/CA without CTS/RTS
A node listens for signals on the network to determine if another node is transmitting. If another node is transmitting, it waits a random amount of time before repeating this process. When it detects that the channel is idle, it will begin the transmission
What hardware is required for WiFi?
A computer needs a wireless network adaptor, which it uses to communicate with a wireless network adapter. The WAP is connected to a router
How is SSH used for remote management?
In order to access a remote computer, SSH requires a username / password combination and encrypts information during transmission. SSH clients are pieces of software which can be used to make a TCP connection to a remote port. Once a connection is established, commands for application-level protocols can be sent to the remote computer.
