IPv4 is tiny by design
All of IPv4 (4.29 billion addresses) could fit inside a single /8 of IPv6—less than one 256th of one percent of IPv6 space.
Space comparison
IP Subnet Calculator (IPv4 & IPv6)
Network & Actions
Enter IP/prefix. Works with IPv4 and IPv6 (IPv4-embedded IPv6 supported).
Results
Results will appear here.
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Understanding IP Subnets (Beginner-Friendly Guide)
This short guide explains the terms you’ll see in the results and how to choose a subnet that fits your needs.
What is CIDR?
CIDR writes a network as IP/prefix, e.g., 192.168.1.10/24 (IPv4) or 2001:db8::/48 (IPv6). The prefix counts the network bits.
IPv4 vs IPv6 at a glance
- IPv4: 32-bit addresses like 203.0.113.25. Has broadcast and “usable hosts”.
- IPv6: 128-bit addresses like 2001:db8::1. No broadcast (uses multicast). Huge address space.
How usable hosts are calculated (IPv4)
For most prefixes, usable hosts = total − 2 (reserve network & broadcast). Special cases:
- /31: point-to-point — both addresses usable.
- /32: single host.
Choosing a prefix length
- Smaller number (e.g., /16) → larger network.
- Larger number (e.g., /28) → smaller network.
- IPv6 best practice: LANs use /64; sites often receive /48 or /56.
Quick References
IPv4
| Prefix | Subnet Mask | Usable Hosts | Total |
|---|---|---|---|
| /24 | 255.255.255.0 | 254 | 256 |
| /25 | 255.255.255.128 | 126 | 128 |
| /26 | 255.255.255.192 | 62 | 64 |
| /27 | 255.255.255.224 | 30 | 32 |
| /28 | 255.255.255.240 | 14 | 16 |
| /29 | 255.255.255.248 | 6 | 8 |
| /30 | 255.255.255.252 | 2 | 4 |
| /31 | 255.255.255.254 | 2* | 2 |
| /32 | 255.255.255.255 | 1* | 1 |
*Special cases: /31 point-to-point (2 usable), /32 single host.
IPv6
| Prefix | Addresses | Typical use |
|---|---|---|
| /48 | 280 | Large site allocation |
| /56 | 272 | Small site allocation |
| /64 | 264 | Standard LAN / SLAAC networks |
| /128 | 1 | Single interface address |
5 Fun Facts about IP Subnets
/64 is the IPv6 magic number
IPv6 LANs are almost always /64 because SLAAC (stateless autoconfig) assumes 64 host bits for making addresses from MACs.
Auto addressing
Broadcast is an IPv4 thing
IPv6 dropped broadcast entirely and leans on multicast groups instead (e.g., ff02::1 for “all nodes”).
Multicast life
/31 saves addresses
Point-to-point IPv4 links often use /31 so both addresses are usable—no wasted network or broadcast slot.
Link efficiency
World per-capita addresses
IPv6 offers about 4.8×1028 addresses per human on Earth right now; IPv4 offers fewer than one per person.
Scale shock