ERC-721 Non-Fungible Token Standard

Introduction

What Is a Non-Fungible Token (NFT)?
A non-fungible token (NFT) uniquely identifies an asset or individual, making it ideal for digital collectibles, access keys, lottery tickets, concert seats, and more. Unlike interchangeable cryptocurrencies, each NFT is distinct—valuable for its rarity, age, or visual traits. Enter ERC-721, the gold standard for NFTs.

What Is ERC-721?
ERC-721 establishes a protocol for NFTs on Ethereum. Each token has a TokenId (uint256), ensuring global uniqueness when combined with its contract address. DApps can leverage this to render dynamic visuals—think CryptoKitties, virtual weapons, or art!

Prerequisites

Deep Dive into ERC-721

Proposed in January 2018 by William Entriken et al., ERC-721 standardizes NFTs via a smart contract API. Key features include:

Token transfers between accounts
Balance and ownership queries
Approval mechanisms for third-party transfers

Core Methods

function balanceOf(address _owner) external view returns (uint256);
function ownerOf(uint256 _tokenId) external view returns (address);
function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;
// ... (see full list in EIP-721)

Key Events

event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);
event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);

Example: Querying CryptoKitties

from web3 import Web3

w3 = Web3(Web3.HTTPProvider("https://cloudflare-eth.com"))
ck_contract = w3.eth.contract(address="0x06012c8cf97BEaD5deAe237070F9587f8E7A266d", abi=simplified_abi)
print(f"Kitty Balance: {ck_contract.functions.balanceOf('0x...').call()}")

Popular NFT Projects

👉 CryptoKitties – Breed and collect digital cats.
👉 Bored Ape Yacht Club – Exclusive NFT art with community perks.

Sorare: Fantasy football with tradable player cards.
ENS: Decentralized naming for wallets and websites.

FAQs

Q: Can ERC-721 tokens be divided?
A: No—each token is indivisible (e.g., one whole CryptoKitty).

Q: How do I create an ERC-721 contract?
A: Use OpenZeppelin’s template for gas-efficient deployment.

Q: Are NFTs only for art?
A: They also represent tickets, real estate deeds, and identity credentials.