Network Fundamentals & Models

1. Which layer of the OSI model handles physical addressing, node-to-node data transfer, and operates using MAC addresses and switches?

That’s the Data Link Layer (Layer 2).

Its job is node-to-node (device-to-device) delivery on the same local network. It frames the data, handles error detection, and uses physical addressing — the MAC address burned into each network interface.

Why switches belong here: a switch is a Layer 2 device. It reads MAC addresses and forwards frames to the specific port where the destination device lives, rather than broadcasting everywhere.

Layer	Name	Address type
3	Network	IP
2	Data Link	MAC
1	Physical	None (bits)

2. What is the primary operational difference between a Layer 2 Switch and a Layer 1 Hub?

The difference is intelligence.

• Hub (Layer 1) — blindly broadcasts incoming data to every port. No understanding of addresses, just electrical signal repetition.
• Switch (Layer 2) — learns which MAC address lives on which port, builds a MAC address table, and forwards frames only to the intended destination.

The consequences:

• A hub creates one big collision domain — traffic from every port collides, wasting bandwidth.
• A switch creates a separate collision domain per port and keeps traffic isolated.

Example: if 4 devices on a hub talk, all hear everything. On a switch, only the target device receives its frames.

3. In network architecture terminology, what are Nodes and Links?

• Node — any physical device on the network that can communicate: computers, routers, switches, phones.
• Link — the transmission path connecting two nodes: copper cable, fiber optic, or a wireless channel.

[Node A] ─────── link (cable/fiber/wireless) ─────── [Node B]

The link is the medium; the node is the endpoint. A network is simply a collection of nodes joined by links.

4. Which communication mode allows simultaneous bidirectional data transmission between two endpoints?

Full-Duplex.

• Simplex — one-way only (e.g., keyboard → computer).
• Half-Duplex — two-way, but only one side transmits at a time (e.g., walkie-talkie).
• Full-Duplex — both directions simultaneously (e.g., telephone call, modern switched Ethernet).

Simplex:     →  only one direction
Half-duplex: ↔  but one at a time
Full-duplex: ⇄  both at once

Modern switches use full-duplex: a device can send and receive at the same time.

5. Which physical network topology connects all endpoints directly to a single central hardware device, ensuring that an individual cable break only affects a solitary node but creating a single point of failure?

Star Topology.

        [Node]
          │
[Node] ──[Hub]── [Node]
          │
        [Node]

All devices connect to one central hub or switch.

• Good: one cable break only kills that one node.

• Bad: the central device is a single point of failure — if the hub dies, the whole network dies.

• Mesh — every node connects to many others (maximum redundancy).

• Bus — all nodes share one backbone cable.

• Ring — each node connects to exactly two neighbors in a loop.

Star is the standard for modern LANs because the central switch is cheap and failures are easy to isolate.

6. How is a Hybrid Topology defined in enterprise network design?

A hybrid topology combines two or more distinct base topologies into one network.

      Star-Bus hybrid
   ┌────┴────┐        ┌────┴────┐
 [H1] [H2] [H3]  bus  [H4] [H5] [H6]
   (star workgroup)   (star workgroup)

Common examples:

• Star-Bus — star workgroups hanging off a shared bus backbone.
• Star-Ring — star workgroups connected in a ring at the core.

Why use one? Different parts of an organization have different needs. A department might want a star for fault tolerance, while the backbone between buildings is a ring or bus. Hybrid gives you the strengths of each where they matter — at the cost of complexity.

7. What is a Multi-Homed Host in network infrastructure architecture?

A multi-homed host is a device with two or more active network interfaces connected to separate networks at the same time.

      Internet           Internal LAN
        │                     │
   ┌────┴────┐           ┌────┴────┐
   │   NIC 0 │  Server   │ NIC 1   │
   └─────────┘           └─────────┘

Why run multi-homed?

• Redundancy — if one link fails, traffic moves to the other.
• Load balancing — spread traffic across links.
• Routing — a box that bridges two separate networks.

A classic example is a firewall or proxy with one NIC on the internet-facing network and another on the trusted internal network.

8. What is the function of a Pseudo TTY (Teletypewriter) in modern server operating systems?

A pseudo TTY (PTY) is a software-emulated virtual terminal that behaves like a real physical terminal (TTY).

SSH client ──→ [SSH daemon] ──→ PTY (virtual terminal)
                                    │
                              [shell / command interpreter]

Why it’s needed: programs like SSH and Telnet need to talk to a shell (like bash), and the shell expects a terminal device. A PTY fakes that — it presents the shell with a terminal interface while the actual bytes travel over a network connection.

Without PTYs, you couldn’t run interactive programs over SSH. It’s the plumbing that makes remote command-line access work.

9. What is the primary role of a Backbone Network?

A backbone is the high-capacity central pathway that interconnects different network segments — buildings, floors, or geographic regions.

        [Floor 3 switch]──┐
        [Floor 2 switch]──┼──[BACKBONE core]──┼── [Remote site router]
        [Floor 1 switch]──┘                     └── [Data center]

Characteristics:

• High speed (fibre, 10/40/100 Gbps)
• Carries aggregated traffic between segments
• The core of a hierarchical network

Every floor/branch/local network feeds its traffic up into the backbone, which moves it between segments. If the backbone dies, the segments are isolated from each other — so it’s built for redundancy.

10. What are the 7 layers of the OSI model and how does it compare to the TCP/IP model?

The OSI model has 7 layers, top to bottom:

1. Application — user-facing services (HTTP, FTP, SMTP, DNS).
2. Presentation — encoding, encryption, compression.
3. Session — establishes and manages connections/sessions.
4. Transport — end-to-end delivery, segmentation, reliability (TCP/UDP).
5. Network — logical addressing and routing (IP).
6. Data Link — physical addressing, framing, error detection (MAC, switches).
7. Physical — bits over the medium (cables, radio, hubs).

The TCP/IP model condenses this to 4 layers: Application (OSI 5–7), Transport (4), Internet (3, = IP), and Network Access/Link (OSI 1–2). TCP/IP was the practical model that actually got built; OSI was the theoretical ideal.

OSI	TCP/IP	Example protocols
Application	Application	HTTP, DNS, SMTP
Presentation	(merged)	—
Session	(merged)	—
Transport	Transport	TCP, UDP
Network	Internet	IP, ICMP
Data Link	Link	Ethernet, ARP
Physical	Link	Cables, Wi-Fi

The interview one-liner: OSI is the conceptual 7-layer reference; TCP/IP is the 4-layer model the real internet follows. Remember the layer order with a mnemonic like “Please Do Not Throw Sausage Pizza Away” (Physical→Application).

11. What is encapsulation and what are the PDU names at each layer?

Encapsulation is how data gains headers as it moves down the stack — each layer wraps the data from the layer above with its own header.

The PDU (Protocol Data Unit) name changes at each layer:

Data (application)  →   HTTP request
Segment (transport) →   TCP adds port numbers + sequence
Packet (network)    →   IP adds source/dest IP
Frame (datalink)    →   Ethernet adds MAC addresses + FCS
Bits (physical)     →   electrical/optical signals on the wire

So the PDU ladder is data → segment → packet → frame → bits (TCP/IP terms; OSI calls the network PDU a packet and datalink a frame, transport a segment).

On the receiving side, the process reverses: each layer removes its header (decapsulation) and passes the payload up. The interviewer favourite: “what is a TCP segment wrapped in an IP packet wrapped in an Ethernet frame called at each step?” — the answer is the ladder above. Encapsulation is why a single message can traverse many networks without each device needing to understand the other layers.