What does /24 or /64 after an IP address mean?¶
title: "What does /24 or /64 after an IP address mean?" status: "complete" source_document: "https://docs.google.com/document/d/1hwITMqcQql7avlvg8ux7xjBNKXml1FP5EkTjRBu-K8o/edit?usp=drivesdk"
What does /24 or /64 after an IP address mean?¶
An IP address is sometimes written with a number after a slash, such as 192.0.2.40/24 or 2001:db8:1234:5678::10/64.
The number is the prefix length. It shows how many leading bits identify the network or address range. The remaining bits distinguish addresses within that range.
With IPv4, a /24 prefix contains 256 possible address values. For example, 192.0.2.40/24 sits within the range beginning 192.0.2.0 and ending 192.0.2.255. Some of those values may be reserved or unusable as ordinary device addresses, depending on how the network is configured.
The individual address and the network prefix aren't the same thing. 192.0.2.40 identifies one address within the wider 192.0.2.0/24 block. A registration or routing result for the /24 may describe the organisation responsible for the whole range, not the customer or device using that individual address.
IPv6 uses the same prefix-length principle but has much larger address ranges. A /64 normally describes the first 64 bits as the network prefix, leaving another 64 bits for addresses within that network. It therefore represents an extremely large possible address space, not 64 individual addresses.
The prefix shown in a public registration or routing record may also be wider or narrower than the customer allocation. A provider might announce a large block, delegate smaller ranges and assign individual addresses or prefixes to different services and customers.
Preserve the complete individual IP address when investigating an event. Reducing it to the surrounding /24, /64 or another range loses specificity and may group together unrelated addresses, customers or systems.
A prefix length defines the size and boundary of a network range. It doesn't show that every address in that range is used by the same device, subscriber or person.