A subnet is a distinct network within a larger network. In Avalanche terminology, a Subnet is a group of validators responsible for reaching consensus on one or more blockchains, rather than merely an Internet address range.
How Avalanche subnets work
The associated blockchains can use application-specific execution rules, access policies and fee arrangements. This lets an application organize its own network resources instead of sharing every execution constraint with a general-purpose chain. Its security still depends on its validator set and software; affiliation with Avalanche does not automatically give it the security of every Avalanche validator.
Subnets and Avalanche L1s
Current documentation distinguishes legacy Subnets from the newer Avalanche L1 model introduced through the Etna upgrade. Legacy Subnet validators also validate the Primary Network. Under the newer model, L1 validators need not validate the Primary Network and can follow their own validator-management rules, while synchronizing the P-Chain and paying the applicable continuous fee. Older participation requirements therefore should not be applied to every Avalanche L1.
A subnet is not automatically an Ethereum-style rollup, a shard or a private network. Data visibility, permission to participate and cross-network asset transfers depend on the implementation. Independent execution can reduce competition for resources, but interoperability and validator concentration introduce their own operational and security considerations.