Public Blockchain: How Permissionless Networks Work

A public blockchain lets anyone inspect the ledger and submit transactions under the network rules. The useful question is not whether the network stores data in blocks. It is whether independent participants can agree on shared state without one operator granting membership.

The executed SHA-256 linking example below shows one part of that mechanism. It shows why a changed earlier record breaks a later hash reference, while the sections below separate that property from consensus, signatures, and finality.

What makes a blockchain public?

A public blockchain is a permissionless network. You do not need an invitation from a company or consortium to read the chain, create an account, or broadcast a valid transaction.

Ethereum describes public networks as systems where anyone can read or create transactions and where peer consensus decides which transactions enter the shared state. That openness does not mean every participant performs the same job. Wallet users submit signed requests, nodes verify rules, and validators or miners follow the network’s block-production process.

How a transaction becomes shared state

Network software checks a transaction against protocol rules before a block producer proposes it for inclusion.

Sign and broadcast the transaction

A wallet signs a transaction with the account’s private key, then sends it to nodes. Nodes can check the signature and reject requests that violate rules such as an invalid nonce or an attempt to spend unavailable funds.

Order transactions through consensus

Nodes can see valid transactions in a different order, so the network needs a protocol for choosing a block history. Bitcoin’s developer guide describes the chain as an ordered, timestamped public ledger that protects against double spending and changes to earlier transaction records.

The consensus design determines who can propose blocks, how other participants check them, and when the network treats a transaction as sufficiently settled. Proof of work and proof of stake are different answers to that coordination problem, not interchangeable labels for every public blockchain.

Replicate the accepted history

Once the network accepts a block, nodes update their local copy of the ledger. A public chain is therefore more than a shared database dump. Each participating node applies the same validation rules to the history it accepts.

See hash linking catch a changed record

Each block normally carries a reference to the digest of an earlier block. The compact example below links two text records with SHA-256 and then changes the first value.

import hashlib


def digest(text):
    return hashlib.sha256(text.encode()).hexdigest()


first = "A pays B 3 coins"
second = "B pays C 1 coin"
first_hash = digest(first)
second_hash = digest(first_hash + second)
changed_first_hash = digest("A pays B 30 coins")

print("Second block accepts original first hash:", second_hash == digest(first_hash + second))
print("Second block accepts changed first hash:", second_hash == digest(changed_first_hash + second))

Save the file as public_chain_demo.py and run the following command.

python3 public_chain_demo.py

The original digest produces True. Changing the first record produces False because the second block still expects the original parent digest.

This is not a public blockchain implementation. The example does not create a peer-to-peer network, validate signatures, resolve competing blocks, or establish finality. It isolates hash linkage so you can see the integrity check before adding the rest of the system.

Public blockchain versus private blockchain

Public and private networks use similar data structures, yet they start from different trust models. A public chain expects participation from unknown or loosely connected parties. A private chain gives a known organization or consortium control over membership and often over which data each member can read.

Benefits and constraints

Public networks can give independently operated applications a shared transaction history and a common settlement layer. Their open verification model can reduce the need for one party to maintain the only authoritative copy.

That model has costs. Public transaction data may expose activity that a business must keep confidential, consensus can limit throughput or add confirmation time, and transaction fees can vary with network demand.

Choose the network model from the trust boundary

Start by naming who must write records, who must verify them, who may read them, and who can change the rules. If one organization already owns the data and can enforce access control, a conventional database is usually the simpler choice.

Choose a public blockchain when independent parties need a shared state that no single participant should control. Then inspect the chosen network’s validator rules, data visibility, fee model, finality behavior, and developer documentation before you commit an application to it.