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1. IP Addressing: IPv4, Subnets, Public and Private IPs

1.2 IPv4 Structure: Bits, Octets and the Subnet Mask

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Aata IPv4 cha aatla structure baghu. He ekda samjla ki CIDR aani subnetting laghech samjel.

An IPv4 address is 32 bits, written as four octets (8 bits each) in dotted-decimal form. Each octet ranges from 0 to 255.

     192    .    168    .     1     .    10
  11000000   10101000   00000001   00001010
  |<-8 bits->|
  |<---------------- 32 bits ---------------->|

Total possible IPv4 addresses = 2^32 ≈ 4.3 billion — not enough for today's world of phones, laptops and IoT devices. That is the main reason for NAT, private IPs and IPv6.

Every IPv4 address has two parts:

  • Network portion — identifies the network (like the street name).
  • Host portion — identifies the device on that network (like the house number).

The subnet mask tells which bits are network and which are host. Example: 255.255.255.0 means the first 24 bits are network bits.

Why this matters for security

Firewalls, security groups and IDS rules all match on the network part of an address. If you misread a mask, you may allow a whole network when you meant one machine – for example writing /16 instead of /32 opens 65,536 addresses instead of one.

Ravindra Bagale's Tip

Don't be afraid of binary! Many students are scared of 8-bit conversion and give up on subnetting. Just memorise these eight numbers: 128, 64, 32, 16, 8, 4, 2, 1. Every octet is a sum of these numbers. Convert five IPs to binary every day – within a week you'll have the hang of it.

Practice task

Convert 10.20.30.40 and 172.16.5.129 to binary. Then convert 11000000.10101000.00001010.01100100 back to dotted decimal.

Extended teaching notes

Why this lesson matters

Use this lesson to strengthen systems you own: understand the risk, practise safely in a lab, then write and apply the fix.

Apply the fix across stacks

Same class of bug, same idea in every language.

Java / Spring Boot

Parameterise queries, encode output, CSRF tokens, limited Actuator, dependency scanning.

ASP.NET Core

EF Core / parameterised SQL, Razor encoding, HTTPS, Data Protection.

Node.js / Express

helmet, parameterised SQL, npm audit, avoid eval and shell concat.

Python / Django / Flask / FastAPI

ORM, autoescape, pip-audit, Bandit; never shell=True with user input.

PHP / Laravel

PDO/Eloquent, Blade escaping, composer audit.

Go / Ruby

database/sql placeholders / ActiveRecord; html/template / ERB escaping; govulncheck / Brakeman.

nginx / Apache / Tomcat

Hide versions, TLS, security headers, least-open ports.

MySQL / PostgreSQL / MongoDB / Redis

Private networks, auth on, least-privilege users, no 0.0.0.0/0.

AWS / Azure / GCP

Roles not long-lived keys, private data stores, cloud audit logs on.

Mini case study

Suppose we are on a purple-team exercise for an Indian mid-size company (sample data). Red notes a technique from this lesson against a lab clone. Blue must show the control that stops it, the log that would have fired, and the ticket that hardens production. That is the standard this course trains you for.

Defender's worksheet (do this today)

  1. Write the lesson title, date and the exact lab target you are allowed to use.
  2. List three owned systems (VMs, apps, or cloud accounts) where this lesson applies.
  3. Run one safe check from the lesson on a lab system only. Paste the command and output.
  4. Write a two-sentence fix you would give a teammate.
  5. Name one related public advisory (CERT-In, CISA, NVD, or a vendor bulletin) with its date.

Indian workplace (sample data)

Suppose we are a junior analyst at Infosys, TCS, Wipro or a fintech SOC in Bengaluru/Pune. Turn this lesson into one checklist item with an owner, a due date and a place to store evidence (ticket + screenshot).

Global practice

Compare your notes with OWASP, CIS Benchmarks, NIST CSF/IR guidance or a cloud Well-Architected security question. Write five lines on what overlaps.

Everyday analogy

Explain the risk and the fix to a family member without jargon. If they can repeat the warning, you understand it well enough to teach juniors.

Key terms

  • Control: a safeguard (patch, config, process, detection).
  • Finding: a weakness backed by evidence.
  • Remediation: the fix plus how you verified it.
  • Scope: systems you have written permission to touch.
  • Least privilege: only the access the task needs.
  • Defence in depth: several layers so one failure does not end the story.

Common mistakes

  • Skipping written permission "just for a quick scan".
  • Practising on hostel, office or public targets without authorisation.
  • Collecting tool output without writing the fix.
  • Copying attack steps onto production to "verify". Defenders verify with versions, configs and patches.
  • Publishing screenshots that contain secrets, cookies or internal hostnames.

Recap

  • Evidence, fix, owner, date.
  • Own lab only.
  • Fix > spectacle.