This project has been created as part of the 42 curriculum by dporhomo.

Description

This project focuses on practical network configuration, requiring the calculation and assignment of IP addresses, subnet masks, and routing tables to make non-functioning network topologies operational.

Objective

The primary goal of this project is to develop a practical understanding of how network addressing and routing operate at the Network Layer (Layer 3 of the OSI model) through:

• Calculating subnets, broadcast addresses, and valid host ranges using CIDR notation.
• Optimizing IP allocation using Variable Length Subnet Masking (VLSM).
• Configuring static routing tables, default gateways (0.0.0.0/0), and specific network routes.
• Troubleshooting multi-hop routing issues, particularly identifying and fixing "Return Path" failures.
• Understanding the mechanics of switches (Layer 2) versus routers (Layer 3).
• Differentiating between Public and Private IP addresses and understanding their routability rules across the Internet.

Instructions

• Execution: Download and extract net_practice.1.9.tgz. Run ./run.sh to launch the local web server and open the interface. Alternatively, run python3 -m http.server 49242 and navigate to http://localhost:49242. Enter your login to load personal configurations.
• Usage: Use the training tab to practice and the evaluation tab to generate random configurations for grading. Use the interface logs to diagnose missing gateways or invalid IPs.
• Export: Click [Get my config] upon completing a level to download the .json file containing your valid configuration.
• Submission & Evaluation: Place all 10 exported .json files at the root of your Git repository. During the defense, you must successfully configure three random levels under a time limit without external tools, though a basic calculator (bc) is permitted.

Levels Overview

Level	Topology	Core Skills Tested
1	2 public hosts, /24 & /16 subnets	Basic IP addressing and CIDR boundaries.
2	2 private hosts, /27 & /30 subnets	Subnet allocation and minimizing host waste.
3	3 public hosts via a switch	Switch mechanics and shared local broadcast domains.
4	3 public hosts via a router (3 interfaces)	Router interfaces acting as specific subnet gateways.
5	2 public hosts via a router (2 interfaces)	Cross-subnet IP routing and next-hop logic.
6	1 public host via switch, router, Internet	Default routes (0.0.0.0/0) and internet gateways.
7	2 public hosts via 2 separate routers	Multi-hop forward routing and return-path configuration.
8	2 public hosts via consecutively connected routers	Variable Length Subnet Masking (VLSM) and route summarization.
9	3 public subnets via 2 routers	Preventing subnet overlap and handling RFC 1918 public/private IP rules.
10	4 public hosts via 2 routers and a switch	Advanced VLSM, supernetting, and comprehensive network design.

Usage Example: Level 8

In Level 8, the goal is to connect two hosts to the Internet through two chained routers. The challenge involves strict constraints: one host requires a /28 mask, and the Internet node has a fixed return route of /26. The solution requires combining two /28 host networks into a logically adjacent block, allowing the first router to use a single /27 summary route. Additionally, a /30 mask is used to optimize the point-to-point transit link between the two routers, demonstrating efficient IP allocation while solving the "Return Path Trap".

Resources

• Networking Concepts: TCP/IP 4-Layer model, OSI layers, subnet masks, default gateways, routing tables, and switches.
• References:
  1. Subnet IPv4 Calculator
  2. YouTube tutorials: Network Direction
  3. Subnetting Mastery
  4. Wikipedia (IPv4, IP Address)
• AI Usage: I used an AI assistant as a technical sounding board. For this project, it assisted in diagnosing multi-hop return path failures, explaining VLSM mechanics, determining non-overlapping IP boundaries, and distinguishing between public and private IP constraints across the TCP/IP stack.

Implementation Steps

1. Clear all editable fields.
2. Connect hosts to their local router.
3. Connect routers to each other.
4. Set default routes pointing to the Internet.
5. Configure specific return routes from the Internet back to the internal networks.

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Reference Tables

Reserved IP Addresses

Note: Only public IP addresses can be routed on the public Internet. Ensure no private or reserved blocks are leaked outward.

Range	Designation	Description
127.0.0.0/8	Localhost	Used for loopback interfaces.
10.0.0.0/8	Private (Class A)	Large private internal networks.
172.16-31.0.0/12	Private (Class B)	Blocked from the public internet.
192.168.0.0/16	Private (Class C)	Standard small/home private networks.

IP Classes

Class	Range	Typical Scope
A	1 - 126	Large networks
B	128 - 191	Medium networks
C	192 - 223	Small networks
D	224 - 239	Multicast groups
E	240 - 255	Experimental

Networking Models

Layer #	OSI Model	TCP/IP Model	Function / Examples
7	Application	4. Application	Source / Data Generation (HTTP, DNS)
6	Presentation		Formatting / Encryption (JPEG, SSL)
5	Session		Service / Connection Management
4	Transport	3. Transport	Reliability / Ports (TCP, UDP)
3	Network	2. Internet	IP Addresses / Routing
2	Data Link	1. Link	MAC Addresses / Switches
1	Physical		Cables / Hardware / Bits