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Last update: December 14, 2020

# Genesis file

The genesis file contains network configuration items and genesis block parameters.

## Configuration items

The following table lists the network configuration items in the config object specified in the genesis file.

Item Description
Chain ID Chain ID for the network
Milestone blocks Milestone blocks for the network
ethash Specifies network uses Ethash and contains fixeddifficulty
clique Specifies network uses Clique and contains Clique configuration items
ibft2 Specifies network uses IBFT 2.0 and contains IBFT 2.0 configuration items
transitions Specifies block at which to change IBFT 2.0 validators
contractSizeLimit Maximum contract size in bytes. Specify in free gas networks. The default is 24576 and the maximum size is 2147483647.
evmStackSize Maximum stack size. Specify to increase the maximum stack size in private networks with complex smart contracts. The default is 1024.

## Genesis block parameters

The purpose of some genesis block parameters varies depending on the consensus protocol (Ethash, Clique, or IBFT 2.0). These parameters include:

• difficulty
• extraData
• mixHash.

The following table describes the genesis block parameters with the same purpose across all consensus protocols.

Item Description
coinbase Address to pay mining rewards to. Can be any value in the genesis block (commonly set to 0x0000000000000000000000000000000000000000).
gasLimit Block gas limit. Total gas limit for all transactions in a block.
nonce Used in block computation. Can be any value in the genesis block (commonly set to 0x0).
timestamp Creation date and time of the block. Must be before the next block so we recommend specifying 0x0 in the genesis file.
alloc Defines accounts with balances or contracts.

## Milestone blocks

In public networks, the milestone blocks specify the blocks at which the network changed protocol.

Ethereum Mainnet Milestone Blocks

{
"config": {
...
"daoForkBlock": 1920000,
"daoForkSupport": true,
"eip150Block": 2463000,
"eip150Hash": "0x2086799aeebeae135c246c65021c82b4e15a2c451340993aacfd2751886514f0",
"eip155Block": 2675000,
"eip158Block": 2675000,
"byzantiumBlock": 4370000,
"constantinopleBlock": 7280000,
"constantinopleFixBlock": 7280000,
...
},
}


In private networks, the milestone block defines the protocol version for the network.

Private Network Milestone Block

{
"config": {
...
"constantinopleFixBlock": 0,
...
},
}


Tip

When specifying the milestone block for private networks, you only need to specify the latest milestone. It is implied this includes the preceding milestones.

## Fixed difficulty

Use fixeddifficulty to specify a fixed difficulty in private networks using Ethash. This will keep the network’s difficulty constant and override the difficulty parameter from the genesis file.

Example

{
"config": {
...
"ethash": {
"fixeddifficulty": 1000
},

},
...
}


Tip

Using fixeddifficulty is not recommended for use with Ethash outside of test environments. For production networks using Ethash, we recommend setting a low difficulty value in the genesis file instead. Ethash will adjust the difficulty of the network based on hashrate to produce blocks at the targeted frequency.

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