Subnet Calculator — Free, with /1–/32 CIDR Quick-Reference

Instantly calculate the network address, broadcast address, and usable host count from an IPv4 address and subnet mask — free, no sign-up. Includes a /1–/32 CIDR quick-reference table for network design and troubleshooting.

IP Address (IPv4)
Subnet Mask
Network Address {{ networkAddress }}
Broadcast Address {{ broadcastAddress }}
Number of IP Addresses {{ count }}
Range of Allocatable IP Addresses {{ allocatableStart }} - {{ allocatableEnd }}

CIDR Quick-Reference Table (/1–/32)

Prefix Subnet Mask Total Addresses Usable Hosts
/1 128.0.0.0 2,147,483,648 2,147,483,646
/2 192.0.0.0 1,073,741,824 1,073,741,822
/3 224.0.0.0 536,870,912 536,870,910
/4 240.0.0.0 268,435,456 268,435,454
/5 248.0.0.0 134,217,728 134,217,726
/6 252.0.0.0 67,108,864 67,108,862
/7 254.0.0.0 33,554,432 33,554,430
/8 255.0.0.0 16,777,216 16,777,214
/9 255.128.0.0 8,388,608 8,388,606
/10 255.192.0.0 4,194,304 4,194,302
/11 255.224.0.0 2,097,152 2,097,150
/12 255.240.0.0 1,048,576 1,048,574
/13 255.248.0.0 524,288 524,286
/14 255.252.0.0 262,144 262,142
/15 255.254.0.0 131,072 131,070
/16 255.255.0.0 65,536 65,534
/17 255.255.128.0 32,768 32,766
/18 255.255.192.0 16,384 16,382
/19 255.255.224.0 8,192 8,190
/20 255.255.240.0 4,096 4,094
/21 255.255.248.0 2,048 2,046
/22 255.255.252.0 1,024 1,022
/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
/32 255.255.255.255 1 1

Tips

  • /24 (255.255.255.0) is the most common subnet, supporting up to 254 hosts.
  • /32 specifies a single IP address. Used for firewall rules or pinpointing a specific host.
  • Common private IP ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 (RFC 1918).
  • The number of usable hosts is 2(32−prefix length) − 2 (subtracting the network address and broadcast address).

FAQ

The first address in a subnet is reserved as the network address and the last as the broadcast address; neither can be assigned to a host, so two addresses are always subtracted.

/31 (RFC 3021) is used for point-to-point links between routers. /32 identifies a single host and is commonly used in firewall rules or static routing entries.

CIDR notation (e.g. /24) expresses the prefix length as a slash followed by a number, while a subnet mask (e.g. 255.255.255.0) represents the same information as a 32-bit value — they are two ways of describing the same thing.
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Side Note — The IPv4 Exhaustion Problem: The Day 4.3 Billion Addresses Ran Out

IPv4 addresses are 32 bits, providing approximately 4.3 billion addresses. However, the rapid growth of the internet led to IANA (the Internet Assigned Numbers Authority) fully exhausting its pool in February 2011. APNIC, which manages the Asia-Pacific region, also depleted its standard allocation in April of the same year. The transition to IPv6 (128 bits, approximately 3.4×1038 addresses) is now well underway.

The technology that served as a stopgap for IPv4 exhaustion is NAT (Network Address Translation). Home and office routers allow many devices with private IP addresses to share a single global IP, keeping IPv4 viable today. The private IP ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are defined by RFC 1918 and are not directly routed on the internet.

As of 2024, IPv6 adoption rates have reached around 40–50% globally. Meanwhile, dual-stack operation — running IPv4 and IPv6 side by side — continues, and development of interoperability and migration tools remains active.

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