What subnet mask is most commonly used for IPv6 LANs?

The most commonly used subnet mask for IPv6 Local Area Networks (LANs) is /64. This is because a /64 subnet provides a vast address space that is suitable for a single subnet, consisting of 2^64 addresses. This large address pool allows for efficient allocation of addresses and facilitates features like Stateless Address Autoconfiguration (SLAAC), which enables devices on the network to automatically generate their own IP addresses.

Using a /64 subnet helps to adhere to best practices in IPv6 networking, which recommend that individual links or segments of a LAN should have a /64 prefix. This allows for easier management and ensures compatibility with various IPv6 features and protocols, which may expect a /64 subnet to function optimally.

In contrast, a /30 subnet would provide only four addresses, which is insufficient for most LAN environments, while a /128 subnet offers a single address, typically used for individual device addressing rather than LANs. A /16 subnet, although providing more addresses than needed for most LANs, is not the standard practice as it can lead to inefficient use of address space.

Thus, the usage of a /64 subnet aligns with both best practices and the technical requirements of IPv6 addressing within a LAN context.