# CIDR Notation Explained (What Is CIDR?) > CIDR notation explained: what 10.0.0.0/22 means, how CIDR maps to subnet masks, why it replaced classful networking, plus a CIDR-to-mask reference table. - Page URL: https://itinsighthub.com/guides/cidr-notation-explained/ - Markdown variant of this page: append `?format=md` to any URL on this site or send `Accept: text/markdown`. - Full site index for AI assistants: https://itinsighthub.com/llms.txt - Full site export: https://itinsighthub.com/llms-full.txt # CIDR Notation Explained What the slash in `10.0.0.0/22` actually means, how CIDR relates to subnet masks, and why it replaced the old class system. Published August 22, 2026 ## What CIDR is CIDR stands for **Classless Inter-Domain Routing**. It is a notation for writing an IP network as an address followed by a slash and the number of leading bits that belong to the network — for example `192.168.1.0/24`. The part after the slash is called the **prefix length**, and everything it does not cover is host space. The same information used to be expressed as a dotted subnet mask (`255.255.255.0`). CIDR and subnet masks are two spellings of one idea: the prefix counts how many 1-bits are in the mask. ## Where it came from CIDR was published in September 1993 in [RFC 1518](https://datatracker.ietf.org/doc/html/rfc1518) and [RFC 1519](https://datatracker.ietf.org/doc/html/rfc1519). Before that, addresses were handed out in fixed classes — A (/8), B (/16), C (/24). Class B was far too big for most organizations and class C too small, so address space was being consumed at a pace that could not last, and routing tables were filling with thousands of separate class C routes. CIDR let providers allocate blocks of any power-of-two size and aggregate them into single announcements. ## Anatomy of 10.0.0.0/22 Take this real-world-style block apart piece by piece: 10.0.0.0/22 - **Base address (10.0.0.0):** Identifies the network itself. By convention the host bits are all zero. - **Prefix (/22):** The first 22 bits are fixed network bits; the remaining 10 bits are host bits. - **Equivalent mask:** 255.255.252.0 — the same 22 ones written as four octets. - **Range:** 10.0.0.0 – 10.0.3.255 - **Capacity:** 1,024 addresses, 1,022 usable by the traditional rule. A quick sanity check: any /22 spans exactly 1024 addresses, so its third octet advances in steps of 4 — which is why this block ends at `10.0.3.255`. ## CIDR to subnet mask reference (/16–/32) *Prefix lengths, dotted-decimal masks and capacity* | CIDR | Dotted-decimal mask | Total addresses | Usable hosts | | --- | --- | --- | --- | | /16 | 255.255.0.0 | 65,536 | 65534 | | /17 | 255.255.128.0 | 32,768 | 32766 | | /18 | 255.255.192.0 | 16,384 | 16382 | | /19 | 255.255.224.0 | 8,192 | 8190 | | /20 | 255.255.240.0 | 4,096 | 4094 | | /21 | 255.255.248.0 | 2,048 | 2046 | | /22 | 255.255.252.0 | 1,024 | 1022 | | /23 | 255.255.254.0 | 512 | 510 | | /24 | 255.255.255.0 | 256 | 254 | | /25 | 255.255.255.128 | 128 | 126 | | /26 | 255.255.255.192 | 64 | 62 | | /27 | 255.255.255.224 | 32 | 30 | | /28 | 255.255.255.240 | 16 | 14 | | /29 | 255.255.255.248 | 8 | 6 | | /30 | 255.255.255.252 | 4 | 2 | | /31 | 255.255.255.254 | 2 | 2 (RFC 3021) | | /32 | 255.255.255.255 | 1 | 1 (host route) | The inverse direction matters too: Cisco-style ACLs use wildcard masks, where the bits are flipped. Convert either way with the [wildcard mask calculator](https://itinsighthub.com/wildcard-mask-calculator/). ## Why CIDR won: route aggregation The property that saved the internet's routing tables is **supernetting**: when consecutive blocks share a common prefix, they can be announced as one bigger block. Four aligned /24 networks: Supernet example ``` 10.0.0.0/24 10.0.1.0/24 10.0.2.0/24 10.0.3.0/24 ``` together cover exactly the range 10.0.0.0–10.0.3.255, which is a single /22: 10.0.0.0/22 — four routing entries collapse into one. Multiply that across thousands of customers per provider and you get the internet-scale benefit: routers forward on prefixes, not individual networks, and allocations can be sized to actual demand instead of a fixed class. You can reproduce this collapse yourself with the [IP range to CIDR converter](https://itinsighthub.com/ip-range-to-cidr/), which always emits the minimal set of covering blocks. ## The same idea in IPv6 IPv6 kept the slash syntax but dropped dotted masks entirely — there is no IPv6 equivalent of `255.255.255.0`, only prefix lengths. Conventions differ though: a standard LAN subnet is a [/64](https://itinsighthub.com/ipv6/64/), site allocations are commonly /48 or /56, and a single interface address is a [/128](https://itinsighthub.com/ipv6/128/). Analyze any v6 prefix with the [IPv6 subnet calculator](https://itinsighthub.com/ipv6-subnet-calculator/). ## FAQ **Is a /24 the same as 255.255.255.0?** Yes. They are two notations for the identical mask: /24 says "the first 24 bits are network bits," and 255.255.255.0 is those 24 ones followed by eight zeros, written in dotted decimal. **What does /32 mean?** A /32 fixes all 32 bits, so the block is a single address. It appears as a host route in routing tables (for example, a loopback or a device-specific route) and holds exactly 1 usable address. **How many addresses are in a /22?** 1,024 total addresses, of which 1,022 are usable by the traditional rule (network and broadcast excluded). **Can a subnet mask have gaps, like 255.0.255.0?** No. A valid netmask must be a contiguous run of 1 bits followed by 0 bits, so every mask maps to exactly one prefix length. Masks such as 255.0.255.0 are invalid in CIDR and rejected by well-behaved tooling. **Does CIDR apply to IPv6?** Yes — the slash-prefix syntax carries over unchanged. What changes are the working conventions (a LAN subnet is typically a /64) and that IPv6 has no dotted-decimal mask form at all. ## Put it into practice - [IPv4 subnet calculator](https://itinsighthub.com/subnet-calculator/) — full breakdown of any CIDR block you paste in. - [/22 reference page](https://itinsighthub.com/subnet/22/) — ranges, binary math and examples for this exact prefix. - [Subnet cheat sheet](https://itinsighthub.com/subnet-cheat-sheet/) — every prefix from /0 to /32 on one printable page. - [VLSM calculator](https://itinsighthub.com/vlsm-calculator/) — split a block into right-sized subnets using these same rules. --- © 2026 ITInsightHub · [About](https://itinsighthub.com/about/) · [Contact](https://itinsighthub.com/contact/) · [Privacy](https://itinsighthub.com/privacy/)