Networking Key Concepts

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).

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.

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.

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.

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.

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.

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

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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).