Free IP Subnet Calculator
Calculate network details from an IP address and CIDR prefix.
Quick Examples
CIDR Reference Table
How to Use
- Enter an IP address (e.g., 192.168.1.0).
- Select a CIDR prefix (/0 to /32).
- Click Calculate to see network details.
- Click Copy Results to copy all values.
Frequently Asked Questions
What is CIDR notation?
CIDR (Classless Inter-Domain Routing) notation like /24 defines how many bits of the IP address identify the network. A /24 means the first 24 bits are the network part, leaving 8 bits (256 addresses) for hosts.
What is the difference between network and broadcast address?
The network address is the first address in a subnet (all host bits are 0). The broadcast address is the last (all host bits are 1). Neither can be assigned to a host.
Does this support IPv6?
This tool currently supports IPv4 subnetting only. IPv6 subnet calculation may be added in a future update.
A Short History of CIDR and IPv4 Subnetting
When RFC 791 standardised the Internet Protocol in September 1981, IPv4 addresses were carved into rigid classes. Class A allocations gave 16,777,214 hosts, Class B gave 65,534, Class C gave 254, and there was nothing in between. Any organisation needing more than 254 hosts asked for a Class B, even if it only had 1,000 employees, and tens of thousands of addresses were wasted per assignment. By the late 1980s Class B was vanishing fast, the global routing table was outgrowing backbone-router memory, and the 32-bit IPv4 space was being consumed faster than its designers had ever expected. RFC 1518 and RFC 1519 (September 1993) introduced Classless Inter-Domain Routing, pronounced "cider," which let the network/host boundary fall at any bit position and let sixteen contiguous /24s be advertised as a single /20. Allocation finally matched need, the BGP table stopped collapsing under its own weight, and IPv4 exhaustion was pushed roughly seventeen and a half years into the future. RFC 4632 re-issued and obsoleted RFC 1519 in August 2006 and remains the current authority. Three private ranges were carved out by RFC 1918 in February 1996 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), giving every NAT'd home router and corporate firewall a usable address pool that the public internet would never route. RFC 3021 (December 2000) carved out the /31 exception for point-to-point links, saving roughly two IPv4 addresses per router-to-router circuit. The IANA top-level free pool was finally exhausted on 31 January 2011; APNIC followed in April 2011, LACNIC in 2014, ARIN in 2015, AfriNIC in 2017, and RIPE NCC in November 2019. CIDR is the reason any of those dates is in this decade rather than the 1990s.
The Anatomy of a Subnet Calculation
- The 32-bit IPv4 address. Every IPv4 address is 32 bits, conventionally written as four 8-bit octets in dotted decimal.
- The CIDR prefix length. The number after the slash counts the consecutive leading 1-bits in the subnet mask.
- The subnet mask. The dotted-decimal twin of the CIDR prefix.
- The network address. Computed as IP AND subnet_mask bitwise.
- The broadcast address. Computed as network_address OR (NOT subnet_mask) bitwise.
- Total addresses and usable hosts. Total = 2^(32 − prefix), usable hosts = total − 2 (with /31 and /32 exceptions).
Common Subnet Sizes and Where They Show Up
- Home and small-office LANs (/24). Almost every consumer router ships with a /24 default inside 192.168.0.0/16.
- Mid-to-large enterprise networks (10.0.0.0/8 hierarchies). Carve everything out of 10.0.0.0/8 in nested /16, /20, /22, /24 layers.
- Cloud subnets (AWS VPC, Azure VNet, GCP VPC). /16 supernet, /20 or /24 per AZ. AWS reserves 5 addresses per subnet, not 2.
- Point-to-point links (/30 and /31). Router-to-router circuits, tunnels, serial links. RFC 3021 lets a /31 use both addresses.
- DMZs, server VLANs, and small segments (/27 to /29). Internet-facing servers, management VLANs, IoT segments.
- Certification study (CompTIA Network+, Cisco CCNA / CCNP, JNCIA). Subnet calculation is a fixture of every entry-level networking exam.
Key RFCs and Historical Milestones
- RFC 791 (September 1981). Jon Postel's original Internet Protocol specification. Defined IPv4 and the classful allocation scheme.
- RFC 1518 and RFC 1519 (September 1993). Introduced Classless Inter-Domain Routing (CIDR).
- RFC 1918 (February 1996), BCP 5. Carved out the three private IPv4 ranges (10/8, 172.16/12, 192.168/16).
- RFC 3021 (December 2000). The /31 exception for point-to-point links.
- RFC 4632 (August 2006), BCP 122. Re-issued the CIDR specification, obsoleting RFC 1519. Current authority.
- RFC 6598 (April 2012). Reserved 100.64.0.0/10 for Carrier-Grade NAT.
- RFC 6890 (April 2013), BCP 153. Consolidated every IPv4 special-purpose reservation into a single IANA registry.
- IPv4 exhaustion (31 January 2011 and after). IANA pool emptied January 2011; APNIC 2011, LACNIC 2014, ARIN 2015, AfriNIC 2017, RIPE NCC November 2019.
More frequently asked questions
What's the difference between a subnet mask and a CIDR prefix?
They express the same information in two notations. /24 in CIDR is 255.255.255.0 as a dotted-decimal subnet mask.
Why can't I use the network or broadcast address?
The network address is reserved as the identifier for the subnet itself; the broadcast address is reserved for "send to every host."
What about IPv6?
IPv6 uses the same prefix-notation idea but with 128-bit addresses; the standard end-user subnet size is /64.
Is anything sent to a server?
No. The calculator runs entirely in your browser; nothing is transmitted, logged, or stored.
What's the smallest practical subnet?
/30 for typical point-to-point links, /31 (RFC 3021) for the same case with both addresses usable, /32 for single-host routes.
How do I pick a private range for my home or office network?
List every network you'll need to reach (VPNs, VPCs, partner networks) and pick a range that does not overlap any of them.