Notes

Network Knowledge

OSI Network Model

• OSI : The Open Systems Interconnection reference model is a well known Network Model created in the UK that has both conceptual layers and suggested protocols for each.

  • There are seven layers to the OSI Model.
  • Application : Includes information used by client-side software, data from this later interacts directly with applications. (i.e. HTTP)
  • Presentation : The syntax later that converts data from machine-readable to human-readable. Includes data compression, encryption, and character encoding. (i.e. JPEG & GIF)
  • Session : Includes protocols responsible for authentication and data continuity. Includes authorization or re-establishing a dropp connection. (i.e. RPC (Remote Procedure Call))
  • Transport : Utilization of transport protocols (i.e. TCP and UDP)
  • Network : Basically the internet layer (i.e. IP)
  • Data Link : Deal with connections from one machine’s network interface to another. (i.e. ethernet)
  • Physical : Translating raw electrical signals to bits & bytes of data. (i.e. Wi-Fi & DSL)
• It is important to remember that OSI Model is highly conceptual, it’s practical uses are limited.

• TCP/IP is much more practical compared to OSI, but it is purely practical.

TCP/IP Model

• Reference Model : A High-level overview of a complex topic provided by an organization that manages it.

Layers of the TCP/IP Model

• Application : Includes protocols related to user-facing data. Anything that is transmitted from the Transport layer is considered Application Layer Data (i.e. HTTP & FTP)
• Transport : TCP & UDP.
• Internet : Connects separate networks together (IP)
• Link : Lower-level communication standards.
• Physical? : There is a supposed fifth layer that cinludes all the electrical concepts that span across wires, but it is not officially stated.
• We can think of the layers of our reference model as being encapsulated.

Binary and Hexidecimal

• Binary : Number expressed in the base-2 numeral system or binary numeral system.
  • Base : Number System, computers use a Base 2 NS.

• CPU : Central Processing Unit, an electronic circuitry within a computer that executes instructions that make up a computer program.

• It is easy to understand binary numbers if you remember the bases.

• 1, 2, 4, 8, 16, 32, 64, 128…etc.

• Bit : A single digit represented by either 1 (ON) or 2 (OFF).

• Byte : Sequence of 8 Bits.

• Hexadecimal : Base 16, useful numeric system due to it making it easier for us to read binary.
  • Helps to make up for base 10 only going from 0 - 9, adds in A, B, C, D, E, F.
  • You will sometimes see hexadecimals pre-pended with a 0x.
  • FF = 255 = 11111111 = 1 byte

hexadecimal: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,  A,  B,  C,  D,  E,  F
decimal:     0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15

Regular Numbers: 4 8 15 16 23 42
Binary: 00000100 00001000 00001111 00010000 00010111 00101010
Hexadecimal: 04 08 0F 10 17 2A

• If you provide a numerical base in JS in the .toString() method you can convert things to binary and decimal. (parseInt can also be used)

Number(42).toString(16); // 2a
Number(42).toString(2); // 101010

parseInt("101010", 2); // 42
parseInt("2A", 16); // 42

Internet Protocol

• TCP : Transmission Control Protocol.
  • TCP is fault tolerant and end-to-end.
  • If data transmission fails it can be cached and re-sent.
  • No single central system that can take down the entire network.
• IP : Internet Protocol.
• Internet : A series of interconnected networks sharing data.
• Packet : Format that IP data is packaged in.
  • Contains metadata in headers, and body content.

• Pack-Switching : When a message is split up into separate ‘packets’, delivered to a destination, and reassembled as appropriate. > A Byte is 8 Bits >

• IPV4

• IPv4 is still the most commonly used protocol version online.

• IPV6
• Was created because we are running out of IP Addresses.
• Version : Binary representation of the version #.
• Traffic Class : Used to Identify different types of packets.
• Flow Label : An experimental option used for adding packet sequencing into IP.
• Payload Length : Let’s the receivers know how large the data in the packet will be.
• Next Header : Usually identifies the protocol type of the packet’s data.
• Hop Limit : Means of preventing packets from being passed around routers forever.
• Source Address : Where the pack originated.
• Destination Address : Where the packet is heading. > All of the headers have a fixed length of 40 bytes.

Special Addresses

• Local Host : Loopback Address, or the identifier for your current machine.
  • 127.0.0.1
• ALl-Interfaces Address : Used to catch any incoming requests regardless of the intended destination.
  • 0.0.0.0

Transport Protocols

• Transport Protocols act as our delivery person.
• IP is concerned with machine-to-machine communication.
• HTTP is designed for application-to-application communication.
• Port : Virtual interfaces that allow a single device to host lot’s of different applications & services.

• TCP : Transmission Control Protocol, the most common transport protocol.

  • Connection-oriented protocol.
  • Reliable protocol because the segments cannot be lost.
  • Heavy protocol to use (hence, sometimes there is buffering)
• UDP : User Datagram Protocol.
  • Unreliable protocol.
  • Connection-less and provides no verification if data has been received.
  • Up to the sender/recipient to manage expectati

DNS

• DNS : Domain Name System is a distributed appraoch to providing easily-understood names for internetworked devices. (Similar to a phonebook)
  • Invented as a way to distribute the work to numerous organizations, lightening the load and allowing much more rapid growth.
• Domain : A website’s domain refers to the ‘friendly’ name for the website’s host, or the server providing the site’s content.
  • Top Level Domain the last part of the domain (.com, .net, .org)
  • Second Level Domain (Name of the website)
  • Subdomain (www)
• Resolution : Process of working out which name server is needed.
  • During resolving, we work from right to left.

DNS Records

• Zone File : Text file containing, host names, IP Addresses, and resource types.
• Common DNS Record Types:
  • SOA : Start of Authority, let’s use know what name server is the primary authority (THE MINIMUM REQUIREMENT IN A ZONE FILE)
  • NS : Keeps us connected to our zone by pointing to name servers.
  • A/AAAA : A = Domain Name to IPv4 & AAAA = Domain Name to IPv6
  • CNAME : Links Domain name to Domain Name.
  • MX : Mail Exchanger, used by e-mail clients.
  • TTL : Time to live, measure of how long a record should be cached by a DNS name server.

Network Hardware

• HUB : Simplest networking device aka a Signal Splitter.

  • Cheap and found in older networks.
  • Hubs do not do any filtering, every single data packet is sent to every single device all the time.
  • No one uses them anymore.
• Switches: Traffic Control : Intelligent Hubs, track devices connected to them, help manage network load, and can manage separate internal networks with ease.
  • The Max Address Table is the largest difference from hubs.
  • Flood : If a destination address is unknown, the switch will flood received data out to all connect devices.
  • Forward : If the destination is known, it will send data directly to that device.
  • Filter : When data is dropped entirely from a transfer.
• Routers : Connect separate networks with each other.
  • Participate in a process called NAT (Network Address Translation)