# What does each genesis.json parameter mean?

I'm trying to setup genesis.json for my private network. What does each parameter mean and what value can I set in genesis.json?

• nonce: any data?
• timestamp: any time?
• parentHash: any hash?
• extraData: put any data if you wish to add? what is the max size?
• gasLimit: What is the max number?
• difficulty: What is the lowest number?
• mixhash: What is this?
• coinbase: Ethereum address for coinbase, right?
• alloc: first ether allocation

Sample genesis json file from How To Create A Private Ethereum Chain. Official document for private network genesis json is here. Connecting to the network

{
"timestamp": "0x0",
"parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
"gasLimit": "0x8000000",
"difficulty": "0x400",
"mixhash": "0x0000000000000000000000000000000000000000000000000000000000000000",
"coinbase": "0x3333333333333333333333333333333333333333",
"alloc": {
}
}


## Genesis block Explanation

mixhash A 256-bit hash which proves, combined with the nonce, that a sufficient amount of computation has been carried out on this block: the Proof-of-Work (PoW). The combination of nonce and mixhash must satisfy a mathematical condition described in the Yellowpaper, 4.3.4. Block Header Validity, (44). It allows to verify that the Block has really been cryptographically mined, thus, from this aspect, is valid.

nonce A 64-bit hash, which proves, combined with the mix-hash, that a sufficient amount of computation has been carried out on this block: the Proof-of-Work (PoW). The combination of nonce and mixhash must satisfy a mathematical condition described in the Yellowpaper, 4.3.4. Block Header Validity, (44), and allows to verify that the Block has really been cryptographically mined and thus, from this aspect, is valid. The nonce is the cryptographically secure mining proof-of-work that proves beyond reasonable doubt that a particular amount of computation has been expended in the determination of this token value. (Yellowpager, 11.5. Mining Proof-of-Work).

difficulty A scalar value corresponding to the difficulty level applied during the nonce discovering of this block. It defines the mining Target, which can be calculated from the previous block’s difficulty level and the timestamp. The higher the difficulty, the statistically more calculations a Miner must perform to discover a valid block. This value is used to control the Block generation time of a Blockchain, keeping the Block generation frequency within a target range. On the test network, we keep this value low to avoid waiting during tests, since the discovery of a valid Block is required to execute a transaction on the Blockchain.

alloc Allows defining a list of pre-filled wallets. That’s an Ethereum specific functionality to handle the “Ether pre-sale” period. Since we can mine local Ether quickly, we don’t use this option.

coinbase The 160-bit address to which all rewards (in Ether) collected from the successful mining of this block have been transferred. They are a sum of the mining reward itself and the Contract transaction execution refunds. Often named “beneficiary” in the specifications, sometimes “etherbase” in the online documentation. This can be anything in the Genesis Block since the value is set by the setting of the Miner when a new Block is created.

timestamp A scalar value equal to the reasonable output of Unix time() function at this block inception. This mechanism enforces a homeostasis in terms of the time between blocks. A smaller period between the last two blocks results in an increase in the difficulty level and thus additional computation required to find the next valid block. If the period is too large, the difficulty, and expected time to the next block, is reduced. The timestamp also allows verifying the order of block within the chain (Yellowpaper, 4.3.4. (43)).

parentHash The Keccak 256-bit hash of the entire parent block header (including its nonce and mixhash). Pointer to the parent block, thus effectively building the chain of blocks. In the case of the Genesis block, and only in this case, it’s 0.

extraData An optional free, but max. 32-byte long space to conserve smart things for ethernity. :)

gasLimit A scalar value equal to the current chain-wide limit of Gas expenditure per block. High in our case to avoid being limited by this threshold during tests. Note: this does not indicate that we should not pay attention to the Gas consumption of our Contracts.

In addition to the fields described by niksmac:

config Configuration to describe the chain itself. Specifically the chain ID, the consensus engines to use, as well as the block numbers of any relevant hard forks.

The meaning/specification of "config" in genesis.json

type ChainConfig struct {
ChainId *big.Int json:"chainId" // Chain id identifies the current chain and is used for replay protection

HomesteadBlock *big.Int json:"homesteadBlock,omitempty" // Homestead switch block (nil = no fork, 0 = already homestead)
DAOForkBlock   *big.Int json:"daoForkBlock,omitempty"   // TheDAO hard-fork switch block (nil = no fork)
DAOForkSupport bool     json:"daoForkSupport,omitempty" // Whether the nodes supports or opposes the DAO hard-fork

// EIP150 implements the Gas price changes (https://github.com/ethereum/EIPs/issues/150)
EIP150Block *big.Int    json:"eip150Block,omitempty" // EIP150 HF block (nil = no fork)
EIP150Hash  common.Hash json:"eip150Hash,omitempty"  // EIP150 HF hash (fast sync aid)

EIP155Block *big.Int json:"eip155Block,omitempty" // EIP155 HF block
EIP158Block *big.Int json:"eip158Block,omitempty" // EIP158 HF block

// Various consensus engines
Ethash *EthashConfig json:"ethash,omitempty"
Clique *CliqueConfig json:"clique,omitempty"
}

• This completes my answer. +1 – niksmac May 1 '17 at 6:06
• Thanks :-) I think this field might not have existed when you originally wrote your very comprehensive answer. – Richard Horrocks May 1 '17 at 7:48
• @niksmac Can you add "minGasLimit" to your list? Also: can "nonce" be any value? – stone.212 Oct 8 '17 at 22:14
• @niksmac A little off-topic but you might know: is there a way to set the "block gas limit" in the genesis file? – stone.212 Oct 8 '17 at 22:30