Subnet Mask Reference Table
Complete /0 through /32 lookup: CIDR ↔ decimal mask ↔ wildcard ↔ host count. Common subnets highlighted.
The complete /0 through /32 lookup table
Every IPv4 subnet mask, with prefix length, decimal mask, wildcard (inverse) mask, total addresses, and usable hosts. Common subnets are highlighted.
| CIDR | Subnet mask | Wildcard | Total addresses | Usable hosts | Common use |
|---|---|---|---|---|---|
/0 | 0.0.0.0 | 255.255.255.255 | 4,294,967,296 | 4,294,967,294 | |
/1 | 128.0.0.0 | 127.255.255.255 | 2,147,483,648 | 2,147,483,646 | |
/2 | 192.0.0.0 | 63.255.255.255 | 1,073,741,824 | 1,073,741,822 | |
/3 | 224.0.0.0 | 31.255.255.255 | 536,870,912 | 536,870,910 | |
/4 | 240.0.0.0 | 15.255.255.255 | 268,435,456 | 268,435,454 | |
/5 | 248.0.0.0 | 7.255.255.255 | 134,217,728 | 134,217,726 | |
/6 | 252.0.0.0 | 3.255.255.255 | 67,108,864 | 67,108,862 | |
/7 | 254.0.0.0 | 1.255.255.255 | 33,554,432 | 33,554,430 | |
/8 | 255.0.0.0 | 0.255.255.255 | 16,777,216 | 16,777,214 | Largest RFC 1918 (10.0.0.0/8) |
/9 | 255.128.0.0 | 0.127.255.255 | 8,388,608 | 8,388,606 | |
/10 | 255.192.0.0 | 0.63.255.255 | 4,194,304 | 4,194,302 | |
/11 | 255.224.0.0 | 0.31.255.255 | 2,097,152 | 2,097,150 | |
/12 | 255.240.0.0 | 0.15.255.255 | 1,048,576 | 1,048,574 | Mid RFC 1918 (172.16.0.0/12) |
/13 | 255.248.0.0 | 0.7.255.255 | 524,288 | 524,286 | |
/14 | 255.252.0.0 | 0.3.255.255 | 262,144 | 262,142 | |
/15 | 255.254.0.0 | 0.1.255.255 | 131,072 | 131,070 | |
/16 | 255.255.0.0 | 0.0.255.255 | 65,536 | 65,534 | 192.168.0.0/16, /16 enterprise |
/17 | 255.255.128.0 | 0.0.127.255 | 32,768 | 32,766 | |
/18 | 255.255.192.0 | 0.0.63.255 | 16,384 | 16,382 | |
/19 | 255.255.224.0 | 0.0.31.255 | 8,192 | 8,190 | |
/20 | 255.255.240.0 | 0.0.15.255 | 4,096 | 4,094 | |
/21 | 255.255.248.0 | 0.0.7.255 | 2,048 | 2,046 | |
/22 | 255.255.252.0 | 0.0.3.255 | 1,024 | 1,022 | |
/23 | 255.255.254.0 | 0.0.1.255 | 512 | 510 | |
/24 | 255.255.255.0 | 0.0.0.255 | 256 | 254 | Standard home/office subnet |
/25 | 255.255.255.128 | 0.0.0.127 | 128 | 126 | |
/26 | 255.255.255.192 | 0.0.0.63 | 64 | 62 | |
/27 | 255.255.255.224 | 0.0.0.31 | 32 | 30 | 30 hosts — small VLAN |
/28 | 255.255.255.240 | 0.0.0.15 | 16 | 14 | |
/29 | 255.255.255.248 | 0.0.0.7 | 8 | 6 | 6 hosts — small ISP customer |
/30 | 255.255.255.252 | 0.0.0.3 | 4 | 2 | Point-to-point WAN link |
/31 | 255.255.255.254 | 0.0.0.1 | 2 | 2 (point-to-point) | RFC 3021 P-to-P (no network/broadcast) |
/32 | 255.255.255.255 | 0.0.0.0 | 1 | 1 (host route) | Host route (loopbacks, ACLs) |
Quick math reminders
- Each bit added to the prefix halves the subnet. /24 has 256 addresses, /25 has 128, /26 has 64, and so on.
- Usable hosts = total − 2 in most subnets (one for network address, one for broadcast). Exceptions: /31 (P-to-P, RFC 3021) uses both for hosts, and /32 is a single host route.
- Wildcard mask = inverse of subnet mask. Used in Cisco ACLs. /24 = 255.255.255.0 → wildcard 0.0.0.255.
When you actually need each size
| Subnet size | Typical use |
|---|---|
| /30 — 2 hosts | Point-to-point WAN links between routers |
| /29 — 6 hosts | Small ISP customer block, server VLAN |
| /28 — 14 hosts | Small office VLAN, DMZ |
| /27 — 30 hosts | Medium VLAN, branch office |
| /26 — 62 hosts | Larger VLAN, IoT segment |
| /24 — 254 hosts | Standard home or small office — the default |
| /23 — 510 hosts | Floor or department network |
| /22 — 1,022 hosts | Large enterprise floor or campus segment |
| /16 — 65,534 hosts | Whole enterprise, or 192.168.0.0/16 for home labs |
| /12 — 1,048,574 hosts | 172.16.0.0/12 — RFC 1918 mid-range |
| /8 — 16,777,214 hosts | 10.0.0.0/8 — biggest RFC 1918 block |
Don't oversize
Tempting to give every site a /16 "just in case." Don't. Bigger subnets mean bigger broadcast domains, more ARP traffic, harder troubleshooting. A /24 holds 254 hosts — that's already a lot of devices on one segment. If you need more than 254, you probably want multiple smaller VLANs, not one giant /22.