Computer Networks Notes

Network Basics/Overview

• Definition of a Network: A network is a collection of interconnected devices and systems that communicate to share resources, information, or services.
• In computing and telecommunications, a network typically comprises computers, servers, routers, switches, and other devices connected to facilitate data transmission and communication.
• Network Span:
  • Local Area Network (LAN): Spans across an office.
  • Metro Area Network (MAN): Spans across a city.
  • Wide Area Network (WAN): Spans across cities and provinces.

Why Use Computer Networks?

• Computer networks are used for various purposes and provide numerous benefits.
• Reasons for using computer networks:
  • Resource Sharing: Sharing network printers and network cameras, for example.
  • Communication
  • Data Transfer
  • Remote Access
  • Cost Savings
  • Scalability
  • Improved Productivity
  • Global Connectivity

Components in a Computer Network

• Server:
  • A computer program or device that provides functionality for other programs or devices, known as "clients."
• Hosts:
  • Can include computers, servers, routers, switches, printers, and any other network-capable device.
  • Hosts can initiate communication (as a client) and respond to communication requests (as a server), depending on the context.
• Client:
  • A type of host that initiates communication requests or services from servers within a network.
  • Often end-user devices like computers, smartphones, tablets, or IoT devices that request resources from servers.
• NIC (Network Interface Controller):
  • A computer hardware component that connects a computer to a computer network.
  • Also known as a network interface card, network adapter, LAN adapter, or physical network interface.

MAC Address

• Definition: A Media Access Control (MAC) address is a unique identifier assigned to a network interface controller (NIC) for network communication within a network segment.
• Format: Recognizable as six groups of two hexadecimal digits, separated by hyphens, colons, or without a separator.
• Assignment: Primarily assigned by device manufacturers.
• Synonyms: Commonly known as an Ethernet hardware address, hardware address, or physical address.
• Storage: Stored in hardware (e.g., card's read-only memory) or by a firmware mechanism.
• MAC Address Details (Example from the Transcript):
  • Physical Address: D8-0F-99-73-B0-B3
• Note: Many network interfaces support changing their MAC address.

Hub

• A multiport repeater used to connect hosts in a LAN segment.
• Drawback: Due to low throughput, hubs are now rarely used.
• OSI Model Layer: Works on Layer-1 (Physical Layer) of the OSI Model.

Switch

• A multiport bridge used to connect hosts in a LAN segment.
• Speed: Much faster than hubs and operates on wire speed.
• OSI Model Layer: Works on Layer-2 (Data Link Layer), but Layer-3 (Network Layer) switches are also available.
  *Console Cable
  *Layer 3 Switch

Router

• A Layer-3 (Network Layer) device that makes routing decisions for data sent to a remote destination.
• Connected to two or more data lines from different IP networks.
• Function: Reads the network address information in the packet header to determine the destination and directs the packet to the next network based on its routing table or policy.

LAN vs. WAN

Firewall

• Software or a combination of software and hardware used to protect user data from unintended recipients on the network/internet.
• Function: Monitors and controls incoming and outgoing network traffic based on predetermined security rules.
• Establishes a barrier between a trusted internal network and an untrusted external network (e.g., the Internet).

Types of Communication (Duplex)

• Half-Duplex:
  • Two-way communication (send/receive), but only one at a time.
• Full-Duplex:
  • Two-way communication (send/receive) at the same time.

Network Topologies

• Physical Topology:
  • The physical arrangement of the network, including how cables and devices are connected.
• Logical Topology:
  • The path that data signals take through the physical topology.

Bus Topology

• Description:
  • One main cable to which all nodes are directly connected.
  • The main cable acts as a backbone for the network.
• Advantages:
  • Easy and cheap to set up.
  • Requires a small amount of wire.
• Disadvantages:
  • If the main cable breaks, the entire network goes down.

Ring Topology

• Description:
  • Nodes connected in a circle.
  • Data travels in one direction.
  • Tokens are used to transmit data (only the computer with the token can send data).
• Disadvantages:
  • If one workstation shuts down, the entire network is affected.

Star Topology

• Description:
  • Each computer has a cable connected to a single central point.
  • All nodes connect to a hub.
  • All data must pass through the hub, which sends the packet to the destination.
• Advantages:
  • Easy to set up.
  • One cable failure does not crash the network.
• Disadvantages:
  • If the hub goes down, the entire network goes down.

Mesh Topology

• A single link failure will not bring the network down.
• Very expensive due to multiple links between sites and routers.
• Full-Mesh: All computers are connected together, like the Internet, and data will always be delivered.
  • Advantages:
    • All computers connected together.
    • Data will always be delivered.
  • Disadvantages:
    • Lots of cable required.
    • Hard to setup.
• Partial-Mesh: A trade-off between fault tolerance and cost (some routers are not connected to all other routers).

Mesh Topology - Number of Links

• The number of mesh links (w) can be determined using the formula: $w = \frac{n * (n - 1)}{2}$, where:
  • $w$ is the number of WAN (wide area network) links.
  • $n$ is the number of sites.