Ethereum networks (mainnet, testnet) in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Ethereum mainnet?

The Ethereum mainnet is the primary public blockchain where real transactions happen using real Ether (ETH). It is the live network where users send and receive actual value.

beginner

What is a testnet in Ethereum?

A testnet is a separate Ethereum network used for testing and development. It uses fake Ether and allows developers to try out smart contracts without risking real money.

intermediate

Name two popular Ethereum testnets.

Two popular Ethereum testnets are Goerli and Sepolia. They help developers test their applications safely before deploying on the mainnet.

beginner

Why should developers use testnets before mainnet?

Developers use testnets to avoid losing real money and to find bugs or errors in their smart contracts. Testnets provide a safe space to experiment and fix issues.

intermediate

How does the Ethereum mainnet differ from testnets in terms of transaction cost?

On the mainnet, transactions cost real Ether as gas fees. On testnets, gas fees are paid with test Ether, which has no real value and can be obtained freely from faucets.

What is the main purpose of an Ethereum testnet?

A To send real Ether to other users

B To replace the Ethereum mainnet

C To mine real Ether coins

D To test smart contracts without using real Ether

Testnets allow developers to test smart contracts safely using fake Ether.

Which network uses real Ether for transactions?

A Goerli testnet

B Ethereum mainnet

C Rinkeby testnet

D Sepolia testnet

The Ethereum mainnet is the live network where real Ether is used.

What do you call the fake Ether used on testnets?

A Test Ether

B Real Ether

C Bitcoin

D Gas tokens

Test Ether is used on testnets and has no real monetary value.

Which of these is NOT a reason to use an Ethereum testnet?

A To avoid losing real money

B To find bugs in smart contracts

C To deploy a live application for users

D To experiment safely

Testnets are for testing, not for live user applications.

How can developers get test Ether for testnets?

A From faucets that distribute free test Ether

B By mining on the mainnet

C By buying it on exchanges

D By sending real Ether to testnet addresses

Faucets provide free test Ether to developers for testing.

Explain the difference between Ethereum mainnet and testnets.

Think about real vs fake money and purpose of each network.

Why is it important to test smart contracts on testnets before deploying on mainnet?

Consider risks and safety in software development.

Practice

1. Which Ethereum network should you use to test your smart contracts without risking real money?

easy

A. Private network

B. Mainnet

C. Testnet

D. Production network

Solution

1. Step 1: Understand the purpose of testnets

   Testnets are designed for testing and development without real value at risk.

2. Step 2: Compare with mainnet

   Mainnet is the live network where real transactions and value exist, so testing there risks real money.

3. Final Answer:

   Testnet -> Option C

4. Quick Check:

   Testnet = Safe testing network [OK]

Hint: Testnets are for safe practice, mainnet is for real use [OK]

Common Mistakes:

• Confusing mainnet with testnet
• Thinking private network is the default test environment
• Assuming production network is safe for testing

2. Which of the following is the correct way to specify the Ethereum mainnet network ID in your configuration?

easy

A. "networkId": 1

B. "networkId": 3

C. "networkId": 42

D. "networkId": 4

Solution

1. Step 1: Recall Ethereum network IDs

   Mainnet uses network ID 1, testnets have other IDs like 3 (Ropsten), 4 (Rinkeby), 42 (Kovan).

2. Step 2: Match the ID to mainnet

   Network ID 1 corresponds to the Ethereum mainnet.

3. Final Answer:

   "networkId": 1 -> Option A

4. Quick Check:

   Mainnet networkId = 1 [OK]

Hint: Mainnet always uses network ID 1 [OK]

Common Mistakes:

• Using testnet IDs for mainnet
• Confusing networkId with chainId
• Assuming networkId 0 is valid

3. Given the following code snippet to connect to an Ethereum network, what network will it connect to?

const provider = new ethers.providers.JsonRpcProvider("https://rinkeby.infura.io/v3/YOUR_PROJECT_ID");
const network = await provider.getNetwork();
console.log(network.name);

medium

A. "mainnet"

B. "rinkeby"

C. "ropsten"

D. "kovan"

Solution

1. Step 1: Identify the RPC URL

   The URL contains "rinkeby.infura.io", indicating the Rinkeby testnet.

2. Step 2: Understand provider.getNetwork()

   This method returns the network info based on the RPC URL, so it will return "rinkeby".

3. Final Answer:

   "rinkeby" -> Option B

4. Quick Check:

   RPC URL domain = rinkeby [OK]

Hint: Check the RPC URL domain for network name [OK]

Common Mistakes:

• Assuming default is mainnet
• Confusing testnet names
• Ignoring the URL and guessing

4. You wrote this code to deploy a contract on Ropsten testnet but get an error:

const provider = new ethers.providers.JsonRpcProvider("https://mainnet.infura.io/v3/YOUR_PROJECT_ID");
const signer = provider.getSigner();
// deploy contract code here

What is the main issue?

medium

A. Incorrect contract deployment syntax

B. Provider does not support getSigner()

C. Using mainnet RPC URL instead of Ropsten testnet URL

D. Missing private key for signer

Solution

1. Step 1: Examine the signer line

   JsonRpcProvider.getSigner() fails because public RPC providers like Infura do not expose signing accounts or private keys.

2. Step 2: Correct approach for deployment

   Create a wallet signer: const wallet = new ethers.Wallet('0x-your-private-key', provider); const signer = wallet.connect(provider);

3. Final Answer:

   Missing private key for signer -> Option D

4. Quick Check:

   Use Wallet(privateKey, provider) for signing [OK]

Hint: Use ethers.Wallet(privateKey, provider) for signing deployments [OK]

Common Mistakes:

• Using provider.getSigner() without wallet
• Assuming getSigner() works without wallet
• Overlooking private key requirement

5. You want to write a script that deploys a smart contract first on the Goerli testnet and then on the mainnet only if tests pass. Which approach correctly handles this?

hard

A. Deploy on Goerli testnet, run tests, then deploy on mainnet if tests pass

B. Deploy on mainnet first, then test on Goerli

C. Deploy simultaneously on both networks

D. Deploy only on mainnet and skip testnet

Solution

1. Step 1: Understand deployment best practices

   Testnets like Goerli are used to test contracts safely before mainnet deployment.

2. Step 2: Apply conditional deployment logic

   Deploy on Goerli, run tests, and only deploy on mainnet if tests succeed to avoid costly errors.

3. Final Answer:

   Deploy on Goerli testnet, run tests, then deploy on mainnet if tests pass -> Option A

4. Quick Check:

   Test on testnet before mainnet deployment [OK]

Hint: Test on Goerli first, then deploy mainnet if tests pass [OK]

Common Mistakes:

• Deploying on mainnet before testing
• Skipping testnets entirely
• Deploying on both networks at once