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

NetPractice 🌐

This project focuses on understanding how networks function at a fundamental level through hands-on problem solving.

Description

NetPractice is a networking project that introduces the basics of computer networks.

The goal is to understand how devices communicate by learning how to configure IP addresses, use subnet masks, connect networks through routers, and understand the role of a default gateway.

This project is completed through a training interface in which each level presents a network configuration problem. The objective is to solve each problem by making the network function correctly.

Instructions

Run the training interface

python3 -m http.server 49242

You may change the port number if needed.

Then open your web browser and go to the URL:

http://localhost:49242

Export configurations

For each level, export the configuration file by clicking the Get my config button.

Submission requirements

There are 10 levels in the training interface, so you must submit 10 configuration files, one for each level.

Requirements:

  • Place all 10 files at the root of the repository
  • Enter your login in the training interface before exporting the files

Networking Concepts & Resources

TCP (Transmission Control Protocol) / IP (Internet Protocol) Addressing

TCP/IP addressing is the system used to identify devices on a network and route data between them. It is part of the TCP/IP model, which is the foundation of how the Internet works.

In simple terms:

TCP/IP addressing = How devices get an address (IP) + how data knows where to go (routing using that address)

IP Address = "Where am I?"

An IP address uniquely identifies a device on a network.

Example:

192.168.1.10

You can think of it like a home address:

  • 192.168.1 -> the network
  • .10 -> the host (device)

Network Type

Network type refers to:

Whether an IP address can be used on the global Internet or only inside a local network

Two Types

  1. Private IP (Internal use only)
  • Cannot be used on the Internet
  • Only used inside local networks (LAN)

Ranges

Range CIDR
10.0.0.0 - 10.255.255.255 /8
172.16.0.0 - 172.31.255.255 /12
192.168.0.0 - 192.168.255.255 /16

Example

Your home WiFi:

192.168.1.10

-> This works inside your house

-> But the Internet does NOT know how to reach it

  1. Public IP (Internet-routable)
  • Can be routed on the Internet
  • Globally unique

Example

8.8.8.8
163.172.x.x
54.44.x.x

-> These exist on the real Internet

Key Difference

Feature Private IP Public IP
Reachable from Internet No Yes
Used in LAN Yes Usually no
Globally unique No Yes

Private IPs work only inside local networks; public IPs are required for communication over the Internet.

TCP/IP addressing includes:

  • IP address
  • Subnet mask
  • Network address
  • Broadcast address
  • Default gateway
  • Routing rules

TCP/IP addressing is how devices are identified and how data is routed across networks using IP addresses and subnet logic.

Useful resources:

Subnet Mask

A subnet mask tells us which part of an IP address represents the network and which part represents the host.

Example:

IP address:							   192.168.1.10
Subnet mask:						   255.255.255.0
CIDR (Classless Inter-Domain Routing): /24

For /24:

  • Network part: first 24 bits
  • Host part: last 8 bits

Subnet masks are essential for determining:

  • the network address
  • the broadcast address
  • the valid host range
  • whether two devices are in the same subnet

Typical subnets for IPv4

CIDR Subnet mask Usable hosts per subnet
/32 255.255.255.255 0
/31 255.255.255.254 0
/30 255.255.255.252 2
/29 255.255.255.248 6
/28 255.255.255.240 14
/27 255.255.255.224 30
/26 255.255.255.192 62
/25 255.255.255.128 126
/24 255.255.255.0 254

We must reserve 2 IP addresses that cannot be used by any device

  • The first IP in the range is reserved to identify the subnet
  • The last IP in the range is reserved for broadcasting messages across all devices in the subnet

/25 -- 2 Subnets -- 126 Hosts/Subnet

Network # IP Range Broadcast
.0 .1-.126 .127
.128 .129-.254 .255

/26 -- 4 Subnets -- 62 Hosts/Subnet

Network # IP Range Broadcast
.0 .1-.62 .63
.64 .65-.126 .127
.128 .129-.190 .191
.192 .193-.254 .255

Broadcast Address

A broadcast address is a special IP address used to send a message to all devices in the same network at once.

Example:

IP address:	 192.168.1.10
Subnet mask: 255.255.255.0 (/24)
  • Network range: 192.168.1.0 - 192.168.1.255
  • Broadcast address: 192.168.1.255

The broadcast address is the last address in the subnet. It is formed by setting all host bits = 1

11111111 = 255
  • You cannot assign a broadcast address to a device
  • It is reserved for network-wide communication

Compare with Other Addresses

Type Example Purpose
Network address 192.168.1.0 identifies the network
Host address 192.168.1.10 identifies a device
Broadcast address 192.168.1.255 send to all devices

Useful resources:

Default Gateway

A default gateway is the device, usually a router, that a computer uses to send data to other networks.

In simple terms:

Default gateway = "the exit door of the local network"

If your computer doesn't know where to send data, it sends it to the gateway.

Example:

IP address: 192.168.1.10
Mask:	 	255.255.255.0 (/24)
Gateway: 	192.168.1.1

This means:

  • the local network is 192.168.1.x
  • the router is 192.168.1.1

How It Works

When you send data:

  1. Check: Is destination in the same subnet?
  2. If YES -> send directly
  3. If NO -> send the packet to the default gateway

Case 1: Same network

192.168.1.20

-> Directly send (no gateway needed)

Case 2: Different network

8.8.8.8

-> Not in your network

-> Send to default gateway (192.168.1.1)

Then:

  • Router takes over
  • Forwards packet to the Internet

How to Find Your Default Gateway

Linux: Type ip route or route -n in the terminal.

Real-Life Analogy

  • Your device = your house
  • Local network = your neighborhood
  • Default gateway = the main road out of your neighborhood

If you go to your neighbor -> no need for main road

If you go to another city -> you must go through the main road

A default gateway is the router your device uses to send data outside its local network.

Useful resources:

Switches and Routers

Switches and routers are both network devices, but they have different roles.

Switch

A switch connects devices within the same local area network (LAN).

What it does:

  • Connects computers, printers, and other devices
  • Sends data inside the same network
  • Uses MAC (Media Access Control) addresses (hardware addresses)

Example:

  • Laptop -> 192.168.1.10
  • Phone -> 192.168.1.20

Devices in the same network can communicate through a switch. No router needed.

Behavior:

  • Learns which device is connected to which port
  • Sends data only to the correct device

Router

A router connects different networks together.

What it does:

  • Connects a local network to other networks or the Internet
  • Decides where data should go
  • Uses IP addresses

Example:

8.8.8.8

This address is not in the local network, so:

-> send to router (default gateway)

-> router forwards it to the Internet

Behavior:

  • Checks the destination IP address
  • Uses a routing table
  • Forwards the packet to next network

Switch vs Router

Feature Switch Router
Scope Same network (LAN) Between networks
Address type MAC address IP address
Main role Connect devices Route traffic
Used for Local communication Internet access
  • Same subnet -> Switch
  • Different subnet -> Router (gateway)

Switch connects devices within a network; router connects different networks and forwards data between them.

Routing Table

A routing table is a set of rules that tells a device: "Where should I send this packet next?"

Basic Structure

  1. Destination
  • The network you want to reach
  • Example: 192.168.1.0/24
  1. Gateway (Next Hop)
  • Where to send the packet next
  • If empty -> send directly
  1. Interface
  • Which network interface to use (eth0, wlan0, etc.)

A routing table usually looks like this:

Destination Gateway Interface
192.168.1.0/24 --(direct) eth0
10.0.0.0/8 192.168.1.1 eth0
0.0.0.0/0 192.168.1.1 eth0

How Routing Works

When sending a packet:

Example:

Send to:

8.8.8.8

Step 1: Check routing table

  • Does 8.8.8.8 match any network? -> No direct match

Step 2: Use default route

0.0.0.0/0

-> Means: "If nothing matches, send here"

Step 3: Forward to gateway

192.168.1.1

Most Important Rule

Routing uses longest prefix match

Meaning:

  • /24 is more specific than /16
  • /16 is more specific than /8

-> Always choose the most specific match

A routing table decides the next hop for a packet based on the destination IP, using the most specific matching rule.

Useful resources:

OSI (Open Systems Interconnection) Layers

The OSI model organizes network communication into 7 layers.

It helps explain how data moves from one computer to another step by step.

The 7 OSI Layers (Top -> Bottom)

Layer Name Function
7 Application User-level apps (browser, email)
6 Presentation Format, encryption, encoding
5 Session Manage connections (start/stop)
4 Transport Reliable delivery (TCP/UDP)
3 Network IP address, routing, default gateway
2 Data Link MAC addresses, switches
1 Physical Cables, electrical or wireless signals

How Data Flows

When sending data:

  1. The application creates the data
  2. Each layer adds its own information (headers)
  3. The data is sent through the network
  4. The receiver processes it layer by layer

Real-Life Analogy

Sending a package:

Step OSI Layer
Write message Application
Put in format Presentation
Start delivery Session
Choose delivery method Transport
Add address Network
Choose local route Data Link
Physically send Physical

Easy Way to Remember

From top to bottom (Application -> Physical): All People Seem To Need Data Processing

OSI layers divide networking into 7 steps, helping us understand how data is sent, addressed, and delivered across networks.

Useful resources:

How AI was Used

AI tools were used as a learning aid to help review networking concepts.

Submission Details

The final submission must include 10 exported configuration files, one for each level, placed at the repository root.

About

Networking fundamentals project from 42 curriculum covering IP addressing, subnetting, and routing configuration.

Topics

networking routing tcp-ip subnetting 42-network ip-addressing

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