When you talk about a /128 prefix, especially in the context of networking and, more specifically, in IPv6, you are referring to a prefix length notation that indicates the subnet mask.

A /128 prefix denotes that all 128 bits of the IPv6 address are used for the network address, leaving 0 bits for host addresses. This is equivalent to a subnet mask of 255.255.255.255 in IPv4, but applied to the much larger addressing space of IPv6.

Practical Meaning

A /128 prefix is ​​generally used to identify a specific, unique IPv6 address. Here are some practical implications of this:

• Unique address: A /128 prefix specifies a single IPv6 address with no room for additional host addresses. Essentially, it identifies a single device or interface on the network.
• Use in Loopback and Point to Point Links: Addresses prefixed with /128 are commonly used for loopback configurations (virtual interfaces within a device) or for point-to-point links (direct connections between two devices), where only one address per interface is needed.
• Security and Administration: Assigning a /128 address can have security implications as it identifies a single network endpoint, and can simplify the administration of certain aspects of the network by knowing that you are only dealing with a single address.

IPv6 and the Use of /128

In IPv6, the use of /128 addresses is quite common due to the vast address space available. Unlike IPv4, where public addresses are scarce and carefully preserved, IPv6 allows the liberal use of unique addresses for specific cases without worrying about address space exhaustion.

Routing Considerations

/128 addresses can also be advertised in routing tables. However, their use must be carefully considered to avoid overloading routing tables with too many individual routes.

In short, a /128 prefix identifies a specific IPv6 address with full precision, with no bit allocation for additional host addresses within that subnet. It is a useful tool for specific use cases that require unique addresses or to simplify the configuration of certain types of network connections.