Networking Key Concepts

Networking Key Concepts

Network Basics

• Home Network:
  • Devices like computers, laptops, mobile phones, and IoT devices (PlayStation, cameras). Connect to a wireless router.
  • The wireless router connects to an Internet Service Provider (ISP).
  • The ISP link connects the home network to the global network.
• Router: A device that connects different networks and directs data packets between them.
• Links:
  • Physical links: Tangible connections.
    • Ethernet cables.
    • Fiber optic cables (buried underground).
    • Coaxial cables (older type).
  • Wireless links:
    • WiFi.
    • Cellular networks (towers).

Network Scale

• Small Business Network:
  • Multiple offices or departments connected via a switch.
  • Example: Offices, printer rooms connected by switches.
• Switch: A device that connects multiple devices (computers, printers, servers) within the same local area network (LAN).
• LAN (Local Area Network): A network connecting devices in a limited area. A home network is also a LAN.
• Medium and Large Networks:
  • Example: University network with different departments or schools.
  • Departments connect to routers, and then to the main gateway connecting to the Internet.
• Worldwide Network: A simplified representation of the Internet, with all networks connected by routers and links.

Network Components

• End Nodes (Hosts): Devices at the end of the communication.
  • Examples: Computers sending or receiving data.
• Intermediate Nodes: Routers and switches that transfer or forward data packets through the network.
• Network Composition: Nodes (end nodes and intermediate nodes) and links.

Network Communication

• The process of exchanging data between devices (computers, phones, servers) connected through a network.
• Example: Host A sends a "hello world" message to Host D.

Message Transfer Methods

• Circuit Switching:
  • A dedicated physical path is established between two devices.
  • Analog phone calls are an example of circuit switching.
• Packet Switching: This is the method used on the Internet.
  • Data is broken into small pieces called packets.
  • Each packet is sent separately through the network and may take different paths to reach the destination.
  • Once all packets arrive, they are reassembled in the correct order to recreate the original message.

Packet Switching Details

• Imagine sending an email from New Zealand to the UK.
• The email is broken into packets, like putting items into several boxes for postal delivery.
• Each packet contains a portion of the data (e.g., four characters) and the destination address.
• Packets may take different routes through the network based on traffic conditions.
• Packets may arrive at the destination out of order, at different times, or some may get lost or corrupted.
• The destination node reorders the packets to recreate the original message.

Addressing

• Computers are identified by their addresses.
• Two Addressing Schemes:
  • IPv4 (Internet Protocol version 4).
  • IPv6 (Internet Protocol version 6).

IPv4

• Uses a 32-bit number.
• The address is represented as four decimal values.
• 2^8 = 256
• Example Binary to Decimal Conversion:
  • 100000002 = 128{10}
  • 100000102 = 130{10}
• Assigned to network interface cards (NIC).
• Logical address (can be changed dynamically).
• Number of possible IPv4 addresses: 2^{32} \approx 4.3 billion.
• Original IPv4 design in 1981 did not foresee the growth of the Internet.

IPv6

• Increases address space using 128 bits.
• Number of possible IPv6 addresses: 2^{128} \approx 3.4 \times 10^{38}, or 340 undecillion addresses.
• Sufficient for every grain of sand on Earth to have an IP address.

Discovering IP Addresses

• Windows: Use ipconfig command.
• The output displays DNS suffixes, IPv6 address, IPv4 address (often a private address), and default gateway (router IP).
• Private addresses (starting with 10) are not visible outside the local network.

Domain Names

• Instead of using IP addresses directly, we use friendly names (domain names) like www.waikato.ac.nz or www.google.com.

DNS (Domain Name System)

• Associates domain names with IP addresses.
• Examples:
  • Facebook's IPv4 and IPv6 addresses.
  • TradeMe's IPv4 addresses.
  • Google's IPv4 and IPv6 addresses.
• The name that is tied to the IP address.
• It's impractical to use the IP address directly, so we assign them names that are friendly.

NSLOOKUP

• A utility to look up the IP address of a domain name.
• On Windows, use nslookup domain_name.
• On Linux/Mac, use host domain_name.
• The query contacts a DNS server, which maintains a mapping table of domain names to IP addresses.
• The DNS server returns the name, address, and aliases associated with the domain.
• DNS servers can query other DNS servers to find mappings if they don't have the entry in their table.
• The process of resolving friendly names to IP addresses is the core function of DNS.
• ISPs typically host DNS servers.

Port Numbers

• A way to differentiate between different services running on a single computer.
• Communication happens with software running on a computer (e.g., web server, email server).
• Port numbers identify the specific service or application.
• Port numbers are 16-bit numbers (0-65535).
• Port numbers 0-1023 are reserved for common protocols and services.
• Higher port numbers are dynamically assigned.
• Examples:
  • 80: Web server (HTTP)
  • 22: Secure Shell (SSH)
  • 21: File Transfer Protocol (FTP)
• Netstat: A utility to view active network connections and the port numbers in use.

Protocols

• Rules and procedures that govern how computers communicate.
• They specify message formats and actions.
• Network protocols often end with "P" (e.g., IP, HTTP).
• Protocols are organized in layers, forming a stack.
  • They add headers and trailers to describe the protocol of that layer.

Protocol Stack (Five Layers)

• Application Layer: HTTP, SMTP, etc.
• Transport Layer: TCP, UDP.
• Network Layer: IP.
• Data Link Layer: Ethernet, Wi-Fi.
• Physical Layer: Cables, wireless signals.
• Each layer communicates with its peer layer on the other host using a specific protocol.
• Data at each layer is encapsulated with headers and trailers relevant to that layer.

Example Data Transmission

• Host A sends an email to Host B.
• The email application divides the data into segments (TCP).
• TCP adds a header to the segment.
• The segment forms an IP packet with an IP header.
• The packet goes to Ethernet and gets an Ethernet header.
• The data is transmitted as a bit stream through the Internet.
• Routers use the IP address to determine the path.
• At the destination, headers are removed layer by layer until the application data is reached.
• Reassembling of packets (if out of order) happens at the TCP layer.
• The operating system implements these protocols.

Wireshark

• A tool to capture and examine network packets.
• Can capture packets traveling through the NIC.
• Allows you to see the headers and contents of packets (though encrypted content will be unreadable).