Introduction to Computer Networks – fundamentals, topologies, IP/subnetting, socket programming, client‑server vs P2P, SDN

1. What is a Computer Network?

A computer network is a set of autonomous computing devices (hosts, routers, switches, etc.) linked by communication links to exchange data, share resources, and provide services. Networks are built on protocols—formal rules that define syntax, semantics, and timing of communication. The most widely used protocol suite is the Internet Protocol Suite (TCP/IP), which maps closely to the OSI reference model.

1.1. Why Networks Matter

• Resource sharing – printers, storage, Internet connection.
• Data communication – email, file transfer, remote login.
• Scalability – adding new devices without redesigning the whole system.
• Reliability – redundant paths increase fault tolerance.

2. Network Types

2.1. Connection‑Oriented vs Connectionless Services

• Connection‑oriented (e.g., TCP) establishes a logical session before data transfer, guaranteeing order, reliability, and flow control.
• Connectionless (e.g., UDP) sends datagrams without prior handshake; lower overhead, suitable for real‑time or loss‑tolerant applications.

3. Network Topologies

Topology describes the arrangement of nodes and links. Two perspectives are common:

graph TD
    A[Bus Topology] -->|Single cable| B[All nodes]
    C[Star Topology] -->|Central hub| D[Nodes]
    E[Ring Topology] -->|Closed loop| F[Token passing]
    G[Mesh Topology] -->|Direct links| H[Redundancy]
    I[Tree Topology] -->|Hierarchical| J[Backbone]
    K[Hybrid Topology] -->|Combination| L[Flexibility]

Classification of physical topologies with key characteristics.

1. Physical topology – actual cabling and device placement.
2. Logical topology – how data flows over the network, independent of physical layout.

3.1. Common Physical Topologies

3.2. Logical Topologies

• Ethernet (CSMA/CD) – logical bus, even if physical star.
• Token Ring – logical ring regardless of physical wiring.
• ATM – cell‑based logical point‑to‑point.

4. OSI Model Overview (relevant to Unit 1)

Only Layer 1 (Physical) and Layer 2 (Data Link) are directly concerned with cabling and media, but understanding the full stack helps in troubleshooting and design.

5. IP Addressing and Subnetting

5.1. IPv4 Address Structure

An IPv4 address is a 32‑bit number, usually written in dotted‑decimal notation:

$$\text{IP}=\underset{\text{four octets}}{\underbrace{a.b.c.d}}=\underset{\text{decimal value}}{\underbrace{a\cdot 2^{24}+b\cdot 2^{16}+c\cdot 2^8+d}}$$

A subnet mask determines the network portion (bits set to 1) and host portion (bits set to 0). CIDR notation (e.g., /24) is a compact way to express the mask.

5.2. Worked Example – Subnetting a Class C Network

Problem: A class C network 192.34.12.0/24 must be divided into three subnets, each supporting at least 30 hosts. Find the new subnet mask, network addresses, and broadcast addresses.

Solution Steps

1. Determine required host bits.
   Minimum hosts per subnet = 30 → need at least $2^h-2\ge 30$.
   $2^5-2=30$ → 5 host bits are required.

2. Calculate subnet bits.
   Class C provides 8 host bits originally.
   Subnet bits = 8 – 5 = 3 bits.

3. New subnet mask.
   Original /24 + 3 subnet bits = /27 → binary mask: 11111111.11111111.11111111.11100000 → 255.255.255.224.

4. Number of subnets.
   $2^3=8$ subnets (more than needed, but acceptable).

5. List first three subnets

Each subnet supports 30 usable hosts (32 total addresses minus network & broadcast).

5.3. Quick Check Questions

• First address (network) and last address (broadcast) for 192.34.12.56/28?
  /28 → mask 255.255.255.240, block size = 16.
  Network = 192.34.12.48, Broadcast = 192.34.12.63.

• Is 192.16.144.64/27 a host, network, or broadcast address?
  /27 block size = 32. Subnet boundaries: 0, 32, 64, 96 … → 192.16.144.64 is the network address of the third subnet.

6. Socket Programming – UDP and TCP

Sockets are the programming interface that abstracts the transport layer. In C (POSIX) the workflow is similar for both protocols; the key differences lie in the socket type and the need for connection establishment.

6.1. UDP (Connectionless) Example (C)

/* udp_server.c – simple UDP echo server */
#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define PORT 5000
#define BUFSZ 1024

int main() {
    int sockfd;
    struct sockaddr_in servaddr, cliaddr;
    char buffer[BUFSZ];
    socklen_t len = sizeof(cliaddr);

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = INADDR_ANY;
    servaddr.sin_port = htons(PORT);

    bind(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr));
    printf("UDP server listening on port %d\n", PORT);

    while (1) {
        int n = recvfrom(sockfd, buffer, BUFSZ, 0,
                         (struct sockaddr *)&cliaddr, &len);
        buffer[n] = '\0';
        printf("Received: %s\n", buffer);
        sendto(sockfd, buffer, n, 0,
               (struct sockaddr *)&cliaddr, len);
    }
    return 0;
}

Diagram (logical flow)

Client (UDP socket) --> sendto() --> Network (IP/UDP) --> recvfrom() --> Server
Server --> sendto() (echo) --> Network --> Client recv()

6.2. TCP (Connection‑Oriented) Example (C)

/* tcp_server.c – simple TCP echo server */
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define PORT 6000
#define BUFSZ 1024

int main() {
    int listenfd, connfd;
    struct sockaddr_in servaddr, cliaddr;
    char buffer[BUFSZ];
    socklen_t len = sizeof(cliaddr);

    listenfd = socket(AF_INET, SOCK_STREAM, 0);
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = INADDR_ANY;
    servaddr.sin_port = htons(PORT);

    bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr));
    listen(listenfd, 5);
    printf("TCP server listening on port %d\n", PORT);

    connfd = accept(listenfd, (struct sockaddr *)&cliaddr, &len);
    printf("Client connected.\n");

    while (1) {
        int n = read(connfd, buffer, BUFSZ);
        if (n <= 0) break;
        write(connfd, buffer, n);   // echo back
    }
    close(connfd);
    close(listenfd);
    return 0;
}

Diagram (logical flow)

Client: socket() → connect() → send() → Server: accept() → read() → write() → Client: recv()

Key Differences Highlighted

7. Communication Paradigms

7.1. Client/Server Model

• Server provides resources/services, runs continuously, listens on well‑known ports.
• Client initiates requests, may be transient.
• Centralized control simplifies security and management but can become a bottleneck.

7.2. Peer‑to‑Peer (P2P) Model

• Every node can act as both client and server.
• Resources are distributed; scalability improves as more peers join.
• Challenges: NAT traversal, security, and consistency.

8. Software Defined Networking (SDN) – A Brief Introduction

SDN separates the control plane (decision making) from the data plane (packet forwarding).

• Controller (e.g., OpenDaylight, ONOS) runs on a general‑purpose server, using southbound APIs (OpenFlow, NETCONF) to program switches.
• Switches become simple forwarding devices that obey flow rules installed by the controller.

8.1. Core Features

1. Centralized network view – global topology and policies.
2. Programmability – network behavior can be changed via software APIs.
3. Abstraction – applications see a virtual network rather than physical hardware.
4. Automation – rapid provisioning, dynamic load balancing, and security enforcement.

8.2. Advantages & Limitations

9. Designing a Small LAN – Application of Concepts

Scenario: An office has 3 departments, each with 50 computers spread over 10 rooms (5 PCs per room).

Design decisions

1. Topology – Choose star per floor with a core‑distribution hierarchy (tree). Each room connects to a switch; all switches uplink to a distribution switch; distribution switches connect to a core router.

2. Media – Use Category 6 twisted‑pair for intra‑room links (up to 1 Gbps) and fiber optic (e.g., 10 Gbps) for backbone between distribution and core to avoid congestion.

3. IP Scheme – Private Class B 172.16.0.0/16. Subnet per department:

   • Dept A: 172.16.0.0/18 → 16 384 hosts (more than enough)
   • Dept B: 172.16.64.0/18
   • Dept C: 172.16.128.0/18

   Each department can be further segmented per floor using /24 subnets if needed.

4. Devices –

   • Switches (Layer 2) for room aggregation.
   • Layer 3 Switch or Router at distribution for inter‑department routing.
   • Wireless Access Points for mobile devices, connected to switches.

5. Security – VLANs per department, ACLs on the router, and optional SDN controller for centralized policy enforcement.

This design illustrates how topology, media choice, IP addressing, and device selection intertwine.

10. Summary of Key OSI Layer Mappings for Common Devices

Exam tip

• Memorize the subnet‑mask calculation steps; many exam questions ask for network, first host, last host, and broadcast addresses. Practice with /28, /27, /24 examples.
• Topologies: be ready to draw a small diagram and list at least two merits and two demerits for each. A comparison table earns quick marks.
• Socket programming: know the sequence of system calls for TCP (socket → bind → listen → accept → read/write) and UDP (socket → bind → recvfrom/sendto). Sketch the logical flow; you don’t need full code, but a short snippet shows depth.
• Client/Server vs P2P: focus on control, scalability, and typical applications; a 2‑column table is a concise answer.
• SDN: remember the three pillars—centralized control, programmability, and separation of planes. A bullet list of features plus one advantage/disadvantage scores well.

By covering definitions, diagrams, worked examples, and comparison tables, you will address all likely marks distribution for Unit 1. Good luck!