Networks

• What is a network

  A network is a linked computer system capable of sharing computer power and resources.
• Process of Internet to the Home

  Backbone cable which is fed across the sea bed, the LSPS( network providers) connect to the backbone and provide to local network providers who distribute it to homes and businesses
• URL

  It gives the full address of an internet resource (name type , file type). When a website is requested with URL the browser requests the corresponding IP address from the local DNS server. If DNS doesn't have the IP address then a larger DNS is searched for. If located the data request is sent to a computer, which finds a webpage containing the host name.
• Internet registrars

  Hold records of existing websites/ domain names available, also sell them to companies willing to purchase.
• Internet registries

  They hold domain names which have already been issued (registrar's domain name, date of registry. They allocate IP addresses associated with part of the DNS system.
• DNS

  Identifies domain name ( each having at least 1 IP address)/ area where an internet resource is stored. Structured in a hierarchy form with smaller domain names written as strings with full stops.
• FQDN(fully qualified domain name)

  Requested by the host then its www.website. Requested by mail of FTP then mail. website.
• WIFI

  Local area technology allows to connect devices through a WAP( wide access point), which has a range of 20m.
• WAP( wireless access point ).

  Needed to connect a device , but you also need a wireless network adapter . Computer + interface card = station . Laptop -WAP-Printer
• Physical bus topology

  Computer connected by a single cable with ends plugged into a terminator

  
  

  Advantage

  • Less cable
  • No hardware requirement so inexpensive

  
  

  Disadvantage

  • If cable fails no data is transmitted to the nodes.
  • Heavier traffic = lower performance
  • Lower security, all computer on network see data transmissions.
• Physical star topology

  centre node which can either be a switch or a computer. The switch records the unique MAC address to identify which computer on the network to send data to.

  
  

  Advantage

  • Multi route so if one cable fails only affects that station.
  • Multi route so less chances of data collision
  • Heavy traffic = constant performance
  • Easy to add new connections

  Disadvantage

  • Cable could be expensive
  • If centre node goes down the network goes down.
• Mesh network topology

  Each node has a connection to every other node, so only one node needs to be connected to the internet for network access.

  
  

  Advantage

  • Can cover entire cities
  • No cabling cost
  • Data isn't needed to travel so faster communication
  • The more node installed the higher the reliability
  • Can go through self-healing - One broken connection can be circumvented by another route.
• Circuit switching

  Creates a direct link between two devices for the duration of the communication. Circuit-switched networks can only connect devices that operate on the same transfer rate

  
  

  Disadvantage

  • Electrical interference could occur as switch are used for connecting and disconnecting a circuit , so data can be lost or corrupt.
• IP address

  Unique address to a network device, which shows where the packet should/ has been sent to/from. The router uses the IP address to direct data packet accordingly
• WAN

  Relies on third-party carrier to spread over large geographical area
• LAN

  Computers on single site/ building connected by cables. User on network can communicate and share data. As it's over a single site, data is transmitted quicker over short distance.
• Physical vs logical topology

  Physical - actual design layout of a network, useful when designing wiring of a new network

  Logical - Path shape data travels in, how component communicate across physical topology
• Packet

  Variable/fixed (500-1500), data transmitted across a network after broken down to manageable chunks
• Packet switching

  Method of communicating across a network in which similar connections are simultaneously happening
• Data packet pathway

  Each has a header(recipient and sender IP which includes hop limit- when data expires /payload(actual data being sent) then has data body. Then the checksum/CRC detects transmission error by checking the hash total calculated contained data.
• Hash total

  Add up all 1's .
• CRC calculated for each packet

  Verifies payload is not corrupt. If corrupt packet is refused and new copy requested from sender
• Packet sending

  Leaves user's computer and takes the fastest route then the header allows it to be reassembled at the recipient's end.
• Routers

  Read the recipient IP address in each packet and forward it to the recipient by the least congested pathway. Routers connect at least 2 networks. The routing table stores and updates the location of the other network devices and the efficient route to them. The routing algorithm finds the optimum route although can become a bottleneck if too much traffic as decisions making is more complicated.
• Gateway

  Used to translate protocol and network, moves data packet to the destination
• MAC Address

  Devices with NIC (network interface card) with hard coded 48bit long, 23 hex written unique code
• Protocol

  Set of rules defining method of data communication
• HTTP (hyper text transport protocol)

  Browser rendering
• TCP/IP

  Four connected layer networks working together passing incoming and outgoing data packet, during network communication.

  
  

  4 Layers

  Application layer

  Transport layer

  Internet layer

  Link layer

  
  

  In each layer , data is wrapped with new packets and descend packets downwards
• Application layer

  Uses protocol of application to transmit over network internet e.g a browser application selects high level protocol (HTTP/FTP)
• Transport layer

  Uses TCP to establish an end-to-end connection with the recipient computer. Data splits into packets and is labelled with packet number - the total number of packets and port number ensures data is handled by the correct application
• Internet layer

  Adds source and destination IP address. Routers use the IP address to forward packets to the destination. IP + Port = Socket
• Socket

  Specifies device packet must be sent to and the application being used on that device
• Link layer

  Physical connection between network nodes. Adds MAC address to the NIC of source and destination computer. Once the packet finds the correct network with IP, it locates the correct hardware. Destination MAC is where the packet is sent next.
• Transferring file with File Transfer protocol

  using the appropriate software on the application layer , the user is presented with a file management screen showing the file and folder structure in the local computer and remote website . Files are transferred by dragging from one area to another . FTP is used in press photographers to upload latest photos
• Role of mail server

  Acts as a virtual post office for incoming and outgoing emails. Stores them until retrievable . The POP3 terrifies email from server and temporarily stores incoming mail . When email received they are transferred to local computer and deleted from server so isn’t always synchronised on different devices . IMAP is an email protocol designed to keep emails in server so mentioning synchronicity . SMTP is used to transfer outgoing emails from one server to another .
• Firewalls

  Security checkpoint between LAN and WAN, preventing unauthorised access between two networks. Which can be software or hardware.

  
  

  Consists of two NICs, one connected to an internal & external network Each packet passes NICs along, and then packets filter.
• Packet filtering

  Controls network access, examining the source and destination of the IP address in the packet header.

  IP matches - data accepted - static filtering happens - Blocks packet based on protocol and port number

  IP mismatched - it's sent back to sender
• Proxy servers

  Intercepts packet entering and leaving networks, hides network address from recipient.

  It maintains the website cache and returns the webpage to users without reconnecting to the internet, speeding up website access.

  Webpage not in cache - Proxy makes own request on behave of user- User IP used - returned data forwarded- cache of page added.
• Virus

  Relies on host file, gets into memory, and infects uninfected files