What a /24 actually gets you: subnet sizes for buyers and lessees
Every listing in the address market is a slash number: a /24 here, a /22 there, priced per address with a size attached. The notation is thirty years old and takes a paragraph to learn, but the sizes carry consequences the paragraph does not mention: what can be routed on its own, what can be split, and what the market will take back off your hands. Here is CIDR for people about to pay for some of it.
The notation in one paragraph
An IPv4 address is 32 bits. The slash number says how many of those bits are fixed as the network part; whatever is left varies, and each leftover bit doubles the count of addresses. So 203.0.113.0/24 fixes 24 bits and leaves 8, giving 256 addresses; a /23 leaves 9 bits for 512; a /22, 1,024; a /16, 65,536. Smaller slash number, bigger block, halving and doubling all the way up and down. That is the entire notation. Everything else about sizes is economics and routing convention.
The sizes that matter
| Block | Addresses | What it is in practice |
|---|---|---|
| /24 | 256 | The atom. The minimum unit to buy, lease or announce on its own. |
| /23 | 512 | Two atoms. The first size that can be split and still route. |
| /22 | 1,024 | The common working size for hosting operations. |
| /20 | 4,096 | Serious infrastructure, still liquid in the market. |
| /16 | 65,536 | An institution. Trades less often, at a per-address discount. |
Anything smaller than a /24 exists inside networks but not really in the market, for the reason in the next section, and anything bigger than a /16 is a negotiation, not a listing.
Why the /24 is the atom
Nothing in the mathematics stops you owning a /25. What stops you is the routing table: by long-standing convention, most networks in the internet's core will not accept an IPv4 announcement more specific than a /24, because carrying every tiny fragment would bloat the global table. A block that cannot be announced on its own cannot live an independent life, so the /24 became the smallest unit that can be bought, leased, routed and, importantly, resold. That single convention explains most of the market's shape: why minimum lease listings are /24s, why a /23 is worth roughly two /24s, and why the same convention is the boundary hijackers aim at, since a /24 wins against anything shorter covering it.
Counting usable addresses honestly
The advertised count is the arithmetic count, and what you can actually put to work is slightly less, depending on use. Inside a routed LAN, the first and last addresses of each subnet are conventionally lost to network and broadcast, and a gateway takes one more, so a /24 used as one flat network yields 253 hosts, and carving it into smaller internal subnets loses a pair per slice. Used as a pool for services, mail addresses, proxy exits or NAT space, nearly all 256 can serve. The honest sizing question is not "how many addresses" but "how many of the way I will use them", and it is worth doing that arithmetic before comparing per-address prices, because a block you must subdivide finely is buying fewer usable addresses than its headline count.
Choosing a size
- Buy the aggregate, not the sum. One /22 is better than four scattered /24s at the same total: it routes as one announcement, splits cleanly later, and sells as either one block or four.
- Size for separation, not just count. Mail wants its streams on separate addresses, customers want assignments that can be reclaimed, and abuse handling wants the ability to isolate a slice without moving everyone. Those needs consume address space faster than raw host counts suggest.
- Mind the per-address curve. Large blocks have lately traded at a per-address discount to small ones, so growing by upgrading one size beats accumulating fragments, and holders of big blocks can earn more leasing them out in /24s than selling whole.
- Leave yourself a resale story. A /24 is the most liquid thing in the market; odd assemblies of fragments are the least. The size you buy is also the size you will one day sell.
Size is also a history question
A /24 offered to you was usually carved out of something bigger, and the parent it came from is part of what you are buying. A slice of a well-run corporate /16 carries its parent's quiet history; a slice of a block that spent years as proxy or spam space carries that instead, and range-level blocklist entries do not care where your slice's paperwork begins. So before comparing prices per address, read the block and its parents: who has held them, what has been flagged inside them, and on which dates. Two /24s with identical arithmetic are routinely worth very different money, and the difference is never in the slash number.
Look up any block on the front page to see its size, its parents, and the history both carry. The report names the source and date behind each fact, and says plainly when nobody has checked something.