A testnet (or test network) is a blockchain network used to run and test blockchains and blockchain projects before releasing to the mainnet. It’s a testing ground for programmers and developers to design, edit, and test their project’s functions and track its performance before making it public. 
Testnet is often used for development. For instance, when creating new functionalities that involve transmitting and receiving cryptocurrency, blockchain testnets serve as accurate replicas of the original protocol, employing identical technology and tools to execute similar tasks. Developers and testers can assess the feasibility of a system by conducting transactions without the risk of actual mainnet funds being at stake. Testnet provides a dynamic and appropriate environment for testing decentralized applications and smart contracts. 
For Ethereum, the two public testnets that client developers maintain are Sepolia and Goerli. Sepolia is a network for contract and application developers to test their applications. The Goerli network lets protocol developers test network upgrades and lets stakers test running validators. 
Quality assurance is vital for deploying high-performing, error-free applications on a network. As organizations adopt a shift-left approach to enhance application quality, test results' validity is significantly influenced by various factors and constraints. Consequently, developers require a test environment that closely resembles mainnet conditions. 
Test network coins are incompatible with main network currencies, requiring new coins or techniques and making the process more prolonged and costly. Nonetheless, all new projects must be tested on a separate blockchain testnet before release, allowing developers to test every component independently and avoiding disruptions in the main network. 
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