Picking the wrong IPv4 block size is a quiet, expensive mistake. You either over-lease and overpay, or under-lease and find yourself back in the market in six months. This is a quick reference to get the decision right the first time.
The Math, Fast
Every IPv4 block size follows the same formula: 2^(32 - prefix length) total addresses, minus 2 reserved (network + broadcast) = usable hosts.
| CIDR | Total IPs | Usable IPs | /24 equivalents |
|---|---|---|---|
| /24 | 256 | 254 | 1 |
| /23 | 512 | 510 | 2 |
| /22 | 1,024 | 1,022 | 4 |
| /21 | 2,048 | 2,046 | 8 |
| /20 | 4,096 | 4,094 | 16 |
| /19 | 8,192 | 8,190 | 32 |
| /18 | 16,384 | 16,382 | 64 |
| /17 | 32,768 | 32,766 | 128 |
| /16 | 65,536 | 65,534 | 256 |
Hard floor to know: /24 is the minimum BGP-routable prefix in the RIPE NCC service region. Sub-/24 blocks are filtered by the vast majority of BGP peers and won't propagate across the public internet. If you only need 30 external IPs, you still lease a full /24.
Cost per IP by Block Size (RIPE Region)
Larger blocks = lower per-IP monthly rate, but the savings curve flattens fast:
| Block | Total IPs | Est. monthly | Per IP/month |
|---|---|---|---|
| /24 | 256 | from ~€180 | ~€0.70 |
| /23 | 512 | from ~€330 | ~€0.65 |
| /22 | 1,024 | from ~€620 | ~€0.61 |
| /21 | 2,048 | from ~€1,150 | ~€0.56 |
| /20 | 4,096 | from ~€2,050 | ~€0.50 |
Prices are illustrative. Actual rates vary by block reputation, availability, and lease term.
The per-IP delta between a /24 and a /22 is small. Lease term length often has a bigger impact on total cost - a 12-month /24 can undercut a month-to-month /22.
Which Block Size for Which Use Case
Shared/web hosting: Start with /24, segment customers per /24 for cleaner blacklist management and abuse isolation.
Dedicated servers (50-100 nodes, multiple IPs each): /22 or /21 - account for IPMI interfaces, management ranges, and customer allocations from the start.
Proxy/VPN rotation pools: Address diversity drives value here. /21 to /20 for production scale; /23 to /22 for pilots. Always check IP reputation history before leasing - it matters more here than anywhere else.
Enterprise multi-site: Minimum one /24 per physical site. Use /22 or /21 aggregates for clean BGP summarization across locations.
Three Things That Trip People Up
1. /22 ≠ guaranteed routing flexibility
A /22 can be announced as a single aggregate prefix. Deaggregating into /24s for traffic engineering is possible, but whether your upstream allows it is a different question. Verify before you commit.
2. Reputation travels with the block
Always check the block's abuse history on Spamhaus, Barracuda BRBL, or via MXToolbox before signing a lease. A cheap /22 with a dirty history will cost you more in operational pain than it saves in rent.
3. RIPE's 24-month transfer lock applies to purchases, not leases
If you buy rather than lease, transferred addresses can't be re-transferred for 24 months under RIPE policy. Leasing sidesteps this entirely.
For the full breakdown - including an interactive IPv4 CIDR calculator, a use-case decision framework, and detailed guidance on lease pricing - check out the complete guide on the IPbnb blog:
IPv4 Subnet Calculator: How to Choose the Right Block Size for Your Project →
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