CIDR Subnet Calculator
Instantaneous IPv4 CIDR prefix decomposition, subnet mask calculation, usable host address range, and bitwise binary diagnostics.
| Subnet Property | Value | Notes & Bit Specifications |
|---|---|---|
| Input IP Address | 192.168.1.100 | Specified host IP within target subnet |
| CIDR Network Block | 192.168.1.0/24 | Canonical routing prefix (base route) |
| Network Address (ID) | 192.168.1.0 | All host bits set to 0 (bitwise AND) |
| Usable Host Range | 192.168.1.1 – 192.168.1.254 | Network ID + 1 through Broadcast - 1 |
| Broadcast Address | 192.168.1.255 | All host bits set to 1 (bitwise OR) |
| Subnet Mask | 255.255.255.0 | 24 leading ones, 8 trailing zeros |
| Wildcard (Inverse) Mask | 0.0.0.255 | Used in Cisco ACLs and OSPF network statements |
| Total IP Addresses | 256 | 2^(32 - 24) = 2^8 total slots |
| Usable Host Count | 254 (99.2% usable) | Total slots minus network ID and broadcast |
| Classful Categorization | Class C | Traditional classful boundary (192.0.0.0/24) |
| IP Scope & Classification | Private (RFC 1918) | Non-routable on public Internet |
| Integer Representation | 3232235876 | 32-bit unsigned decimal integer |
| Hexadecimal IP | 0xC0A80164 | Hex representation (C0.A8.01.64) |
| Binary IP Address | 11000000.10101000.00000001.01100100 | Dotted 4-octet 32-bit binary |
| Binary Subnet Mask | 11111111.11111111.11111111.00000000 | Dotted 4-octet bitmask binary |
This authoritative reference table shows every IPv4 prefix length, the corresponding dotted-decimal subnet mask, wildcard mask, total IP capacity, usable host count, and standard industry applications.
| Prefix | Subnet Mask | Wildcard Mask | Total Addresses | Usable Hosts | Typical Network Scope / Application |
|---|---|---|---|---|---|
| /0 | 0.0.0.0 | 255.255.255.255 | 4,294,967,296 | 4,294,967,294 | Default route / Entire Internet (0.0.0.0/0) |
| /1 | 128.0.0.0 | 127.255.255.255 | 2,147,483,648 | 2,147,483,646 | Half of Internet routing space |
| /2 | 192.0.0.0 | 63.255.255.255 | 1,073,741,824 | 1,073,741,822 | Large CIDR Supernet |
| /3 | 224.0.0.0 | 31.255.255.255 | 536,870,912 | 536,870,910 | Large CIDR Supernet |
| /4 | 240.0.0.0 | 15.255.255.255 | 268,435,456 | 268,435,454 | Large CIDR Supernet |
| /5 | 248.0.0.0 | 7.255.255.255 | 134,217,728 | 134,217,726 | Large CIDR Supernet |
| /6 | 252.0.0.0 | 3.255.255.255 | 67,108,864 | 67,108,862 | Large CIDR Supernet |
| /7 | 254.0.0.0 | 1.255.255.255 | 33,554,432 | 33,554,430 | Large CIDR Supernet |
| /8 | 255.0.0.0 | 0.255.255.255 | 16,777,216 | 16,777,214 | Former Class A network (16.7M hosts) |
| /9 | 255.128.0.0 | 0.127.255.255 | 8,388,608 | 8,388,606 | Class A subnetting |
| /10 | 255.192.0.0 | 0.63.255.255 | 4,194,304 | 4,194,302 | Class A subnetting |
| /11 | 255.224.0.0 | 0.31.255.255 | 2,097,152 | 2,097,150 | Class A subnetting |
| /12 | 255.240.0.0 | 0.15.255.255 | 1,048,576 | 1,048,574 | RFC 1918 Private Class B block (1M hosts) |
| /13 | 255.248.0.0 | 0.7.255.255 | 524,288 | 524,286 | Class A subnetting |
| /14 | 255.252.0.0 | 0.3.255.255 | 262,144 | 262,142 | Class A subnetting |
| /15 | 255.254.0.0 | 0.1.255.255 | 131,072 | 131,070 | Class A subnetting |
| /16 | 255.255.0.0 | 0.0.255.255 | 65,536 | 65,534 | Former Class B network (65.5K hosts) |
| /17 | 255.255.128.0 | 0.0.127.255 | 32,768 | 32,766 | Class B subnetting |
| /18 | 255.255.192.0 | 0.0.63.255 | 16,384 | 16,382 | Class B subnetting |
| /19 | 255.255.224.0 | 0.0.31.255 | 8,192 | 8,190 | Common Cloud VPC Subnet (8K hosts) |
| /20 | 255.255.240.0 | 0.0.15.255 | 4,096 | 4,094 | Common Cloud VPC Subnet (4K hosts) |
| /21 | 255.255.248.0 | 0.0.7.255 | 2,048 | 2,046 | Medium Enterprise Subnet (2K hosts) |
| /22 | 255.255.252.0 | 0.0.3.255 | 1,024 | 1,022 | Standard Large Department (1,022 hosts) |
| /23 | 255.255.254.0 | 0.0.1.255 | 512 | 510 | Dual /24 Supernet (510 hosts) |
| /24 | 255.255.255.0 | 0.0.0.255 | 256 | 254 | Former Class C network (254 hosts) |
| /25 | 255.255.255.128 | 0.0.0.127 | 128 | 126 | Half Class C subnet (126 hosts) |
| /26 | 255.255.255.192 | 0.0.0.63 | 64 | 62 | Quarter Class C subnet (62 hosts) |
| /27 | 255.255.255.224 | 0.0.0.31 | 32 | 30 | Branch office / small VLAN (30 hosts) |
| /28 | 255.255.255.240 | 0.0.0.15 | 16 | 14 | Small cluster / management subnet (14 hosts) |
| /29 | 255.255.255.248 | 0.0.0.7 | 8 | 6 | Point-to-Multipoint / ISP WAN link (6 hosts) |
| /30 | 255.255.255.252 | 0.0.0.3 | 4 | 2 | Traditional Point-to-Point link (2 hosts) |
| /31 | 255.255.255.254 | 0.0.0.1 | 2 | 2 | RFC 3021 Modern Point-to-Point link (2 hosts) |
| /32 | 255.255.255.255 | 0.0.0.0 | 1 | 1 | Single Host / Loopback / Route (1 host) |
| RFC 1918 Block | CIDR Prefix | Subnet Mask | Address Range | Total IPs | Classful Equivalent |
|---|---|---|---|---|---|
| 24-bit block | 10.0.0.0/8 | 255.0.0.0 | 10.0.0.0 – 10.255.255.255 | 16,777,216 | 1 Class A network |
| 20-bit block | 172.16.0.0/12 | 255.240.0.0 | 172.16.0.0 – 172.31.255.255 | 1,048,576 | 16 contiguous Class B networks |
| 16-bit block | 192.168.0.0/16 | 255.255.0.0 | 192.168.0.0 – 192.168.255.255 | 65,536 | 256 contiguous Class C networks |
Special-Purpose & Reserved IPv4 Blocks
| Address Range | CIDR Prefix | RFC Standard | Designated Purpose |
|---|---|---|---|
| 127.0.0.0 – 127.255.255.255 | 127.0.0.0/8 | RFC 1122 | Loopback host addresses (localhost) |
| 169.254.0.0 – 169.254.255.255 | 169.254.0.0/16 | RFC 3927 | Link-Local Auto-Configuration (APIPA) |
| 100.64.0.0 – 100.127.255.255 | 100.64.0.0/10 | RFC 6598 | Carrier-Grade NAT (CGNAT) Shared Address Space |
| 224.0.0.0 – 239.255.255.255 | 224.0.0.0/4 | RFC 5771 | Multicast Address Space (Former Class D) |
| 240.0.0.0 – 255.255.255.254 | 240.0.0.0/4 | RFC 1112 | Reserved for Future Use (Former Class E) |
| 255.255.255.255 | 255.255.255.255/32 | RFC 919 | Limited Broadcast address (local network segment) |
Understanding CIDR (Classless Inter-Domain Routing)
Classless Inter-Domain Routing (CIDR, pronounced "cider") was formally introduced in 1993 by the Internet Engineering Task Force (IETF) in RFC 1518 and RFC 1519 to replace the rigid, inefficient classful network architecture developed in the early 1980s. Under original classful routing, IPv4 networks were strictly carved into Class A (/8, 16.7M hosts), Class B (/16, 65,534 hosts), and Class C (/24, 254 hosts). This inflexible allocation led to severe waste: an organization requiring 300 IP addresses could not fit into a Class C network and was allocated a full Class B block, wasting over 65,000 routable public IPv4 addresses.
CIDR solved this crisis by decoupling IP address assignment from byte boundaries. Instead of enforcing octet boundaries, CIDR allows the network prefix length to be specified at any arbitrary bit boundary from /0 to /32. This introduced two transformative architectural capabilities:
- Variable-Length Subnet Masking (VLSM): Network engineers can subdivide IP spaces into customized subnet sizes tailored precisely to host counts, maximizing address utilization.
- Route Aggregation (Supernetting): Multiple contiguous network prefixes can be aggregated into a single, compact routing table entry. For example, four contiguous
/24subnets can be advertised to upstream BGP peers as a single/22supernet, preventing the global Internet routing table from collapsing under exponential route growth.
How to Calculate Subnetting Math by Hand
Every IPv4 address is an unsigned 32-bit binary integer partitioned into two contiguous segments: the Network ID (the high-order $N$ bits) and the Host ID (the remaining $32 - N$ bits). Understanding the mathematical transformations allows engineers to troubleshoot routing tables, firewalls, and cloud VPC configurations with precision:
1. Computing the Subnet Mask
Given prefix length $N$, the subnet mask consists of $N$ consecutive binary 1s followed by $32 - N$ consecutive binary 0s. To calculate the dotted-decimal value for prefix /24:
11111111.11111111.11111111.00000000 = 255.255.255.0
2. Deriving the Network Address via Bitwise AND
The network identifier is extracted by performing a bitwise logical AND operation between the host IP address and the subnet mask. For each bit position, the output bit is 1 if and only if both the IP bit and mask bit are 1. Any bit paired with a mask bit of 0 is zeroed out. For IP 192.168.1.100 and mask 255.255.255.0, the final octet calculation is 100 & 0 = 0, yielding network address 192.168.1.0.
3. Calculating Total and Usable Host Capacities
With $h = 32 - N$ host bits, the total number of unique binary bit patterns is:
$$ ext{Total Addresses} = 2^h = 2^{32 - N}$$
Under standard RFC rules ($N le 30$), two special addresses are reserved in each subnet:
- Network Identifier: All host bits set to
0. Used to identify the subnet route in IP forwarding tables. - Directed Broadcast Address: All host bits set to
1. Packets sent to this address are delivered to all nodes on the local link layer.
Consequently, the usable host capacity is calculated using the classic formula:
$$ ext{Usable Hosts} = 2^{32 - N} - 2$$
4. Crucial Subnetting Exceptions: /31 and /32
Modern networking standards incorporate two essential exceptions to the $2^{32 - N} - 2$ rule:
- RFC 3021 Point-to-Point Links (/31 Prefix): On point-to-point links between two routers, broadcast capability is unnecessary. RFC 3021 allows a
/31subnet mask (2 total addresses) to provide 2 usable host addresses without reserving network or broadcast IDs, conserving millions of IPv4 addresses across core service provider backbones. - Host Routes (/32 Prefix): A
/32mask assigns a single IP address with zero host bits ($2^0 = 1$). It is universally deployed for virtual loopback interfaces, BGP router identifiers, container endpoints, and VPN client virtual adapters.
Frequently Asked Questions (FAQ)
/24 means the first 24 bits belong to the network identifier (corresponding to 255.255.255.0), leaving the remaining 8 bits to identify individual host machines on that local network segment.
255.255.255.0). A Wildcard Mask (or inverse mask) is the exact bitwise inversion (bitwise NOT) of the subnet mask: 0s mean "must match exactly", while 1s mean "ignore / wildcard" (e.g., 0.0.0.255). Wildcard masks are widely used in Cisco IOS Access Control Lists (ACLs) and OSPF routing protocol configuration statements.
/24 subnet (256 total IPs), only 251 IPs are usable because AWS reserves:
.0 (Network ID), .1 (VPC Router Gateway), .2 (VPC DNS Server), .3 (Future AWS reservation), and .255 (Network Broadcast).
/31, reducing address consumption by 50% compared to legacy /30 links.