# ETHEREUM NETWORKS

#### What Are Ethereum Networks?

Ethereum networks are groups of computers (nodes) running the Ethereum protocol to communicate, process transactions, and keep the blockchain running

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***1. Ethereum Mainnet***
* The main public network
* Real ETH is used
* All live dApps and smart contracts run here
* When people talk about ETH price, they mean Mainnet ETH

***2. Ethereum Testnets***
Used for testing smart contracts and Ethereum upgrades with no real money involved

*Your wallet address works the same across testnets, but:*

* Account balances don’t carry over
* For safety, don’t reuse mainnet wallets on testnets
* It’s safer not to use the same wallet on Mainnet and testnets
* Your ETH balance is not the same.

| Testnet | Purpose | Notes |
| - | - | - |
| **Sepolia** | App & smart contract testing | Default testnet, fast, safe, and recommended |
| **Hoodi** | Protocol & validator testing | Used by core developers & stakers |

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### Popular Testnets:

### **Sepolia**

* Best for developers testing dApps.
* Safe, fast, and recommended for beginners.
* Run by trusted teams (permissioned validators).
* Get free ETH from faucets (e.g., QuickNode, Alchemy, Infura).

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### **Hoodi**

* Good for testing staking and validator stuff.
* Anyone can run a validator (open validator set).
* Has a large state, useful for complex smart contracts.
* Slower to sync, needs more storage.
* Get test ETH from Hoodi faucet or PoW faucet.

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### **Holesky** (Deprecated soon in Sept 2025)

* Was used for validator testing.
* Staking operators now moving to **Hoodi**.

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### 3. **Layer 2 Testnets (For L2 Blockchains)**

These are testnets for **Ethereum scaling solutions**.

***Examples:***

* Arbitrum Sepolia
* Optimistic Sepolia
* Starknet Sepolia

They work on top of Ethereum and help with **faster, cheaper transactions**.

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## 4. **Private Networks**

These are **local or closed Ethereum networks**, like:

### **Development Networks**

* You run this on your own computer.
* Good for quick testing during development.
* Very fast and private.

### **Consortium Networks**

* Used by a group of trusted organizations.
* Like a private Ethereum network shared between schools or companies.

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### My Final Summary a Table

| Type | Purpose | Example |
| - | - | - |
| Mainnet | Real apps & ETH | Ethereum Mainnet |
| Testnets | Test apps safely | Sepolia, Hoodi |
| L2 Testnets | Test Layer 2 dApps | Arbitrum Sepolia |
| Private Networks | Local/controlled testing | Ganache, Hardhat local node |
| Consortium Networks | Shared by trusted orgs | Private Ethereum intranet |

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# Consensus mechanisms

### What Is Consensus in Ethereum?

Consensus just means agreement between many computers (called nodes) on the blockchain

They all must agree on the same truth like what transactions happened and in what order.

***Example:***
If 100 people are keeping records, we only trust the version that most people agree on (at least 66%).

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### What Is a Consensus Mechanism?
A consensus mechanism is the full system (rules, rewards, punishments and protocols) that helps Ethereum nodes:

* Agree on what happened (transactions)
* Keep the network secure
* Reward honest participants
* Punish cheaters

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Ethereum uses Proof of Stake (PoS) now
Let’s look at the 2 main types Ethereum has used:

***1. Proof of Work (PoW):*** Old system used before 2022

* Miners compete to solve math puzzles
* First to solve adds a block and gets ETH
* Uses a lot of electricity
* Secure because hacking it needs 51% of global computing power (almost impossible)

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***2. Proof of Stake (PoS):*** Now used by Ethereum
Validators are chosen randomly (based on how much ETH they staked)

* If chosen they propose a block and get rewarded
* Validators vote on which block is correct
* If they lie their staked ETH is slashed (they lose it)
* More energy-efficient and faste

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#### Extra Concepts (simplified):
1. ***Fork choice rule:*** When there’s more than one possible block, Ethereum picks the one with the most validator support (measured by ETH staked).

2. ***Social coordination:*** In rare cases, the Ethereum community can manually reject a bad chain like a final backup plan

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#### How Do Consensus Mechanisms Keep Ethereum Secure?
Consensus mechanisms are not just about agreement they also protect the network from hackers and bad people trying to cheat the system.

Let’s break it down by type:

**1. Proof of Work (PoW) Security
PoW is secured by computing power**

* To cheat, a hacker needs to control 51% of all mining power in the world
* That means buying super expensive hardware and paying huge electricity bills
* So it’s too costly to attack you’ll lose more than you gain
* Strong security, but wastes a lot of energy.

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**2. Proof of Stake (PoS) Security
PoS is secured by money (ETH).**
* Validators must lock ETH as a "stake"
* If a validator tries to cheat, they lose some or all of their ETH. This is called slashing
* The more ETH you stake the more responsibility and risk if you misbehave

***Very secure because:***

* It’s super expensive to attack (you need to buy lots of ETH then risk losing it)
* Honest validators get rewards dishonest ones get punished

## Sybil Resistance & Chain Selection
#### What is Sybil Resistance?
* It stops fake identities from taking over the network
* In blockchains someone can pretend to be many users (a Sybil attack) to cheat

***To prevent this:***

* Proof of Work (PoW) makes you use electricity/computing power
* Proof of Stake (PoS) makes you lock your money (ETH)
* So, cheating becomes too expensive

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#### What is Chain Selection?
* Sometimes, two blocks come out at once
* The network must choose one correct chain

***This is done using fork choice rules:***

* Bitcoin: picks the longest chain (most work done)
* Ethereum (now): picks the chain with more validator votes (more staked ETH)

***In Summary:***

* Sybil resistance stops fakes
* Chain selection helps everyone agree on the real chain

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### Ethereum’s Gasper = Casper FFG + GHOST
Gasper is the consensus system Ethereum uses after switching to Proof of Stake.

It combines two things:

***Casper FFG (Finality Gadget)***
* Helps finalize blocks once a block is finalized, it can’t be changed
* Validators vote on blocks and help lock them in place.

***GHOST (Fork Choice Rule)***
* Helps nodes choose the best chain if there are multiple.
* It picks the chain with the most staked votes (heaviest one).

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