Read a prefix
01A prefix describes a block of IPv4 addresses. The number after the slash tells you how many of the 32 address bits stay fixed; the remaining bits vary within the block.
Separate comparison: 192.0.2.200/32 selects one address, while 0.0.0.0/0 matches every IPv4 destination. The masks are all ones and all zeros respectively. This is not a usable-host formula.
Source for this drawing: RFC 4632 · Read RFC section 3.1 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
RFC 4632 obsoletes RFC 1519 and consolidates the CIDR address assignment and aggregation plan. This is a publication relationship, not a new IPv4 version or an increase in address width.
Read the updated RFC section ↗A longer prefix fixes more bits and covers fewer addresses.
- IPv4 addresses have 32 bits. CIDR writes an address followed by /n, where n is the number of significant leading bits. A mask has n leading ones followed by zeros; its ones cannot be scattered (§3.1; §5.1).
- A canonical network address has all variable bits zero. 192.0.2.200/25 belongs to network 192.0.2.128/25, whose final octet begins with binary 1. This guide displays canonical blocks, rather than treating any host address as the network base.
- A /24 covers 256 addresses, /25 covers 128, /27 covers 32, /32 covers one, and /0 covers all 4,294,967,296 IPv4 addresses. These are address counts, not a universal count of usable host interfaces. /31 point-to-point use is explained separately in RFC 3021.
- CIDR replaces inference from old class A/B/C boundaries with explicit prefix lengths. Routing protocols and tables need to carry those lengths; the first address octet alone does not tell you the subnet size (§5).
- CIDR helped reduce classful address waste and routing-table growth (§§1–3). It does not create more than 32 bits of IPv4 space or solve address exhaustion. The publication’s allocation figures and dates are historical, not a current availability report.
Fictional documentation block 192.0.2.0/24. Full-width bars represent that block; prefix masks below are actual 32-bit values.
Read the RFC section ↗