CIDR to IP Range Calculator & Subnet Planner
Convert Classless Inter-Domain Routing (CIDR) blocks into accurate network boundaries, usable IP host ranges, broadcast addresses, and Cisco wildcard masks.
Subnet Calculation Breakdown
Subnet Subdivision Planner
Divide this /24 network into smaller equal-sized subnets:
| # | Subnet CIDR | Usable Host Range | Usable Hosts | Action |
|---|---|---|---|---|
| 1 | 192.168.1.0/25 | 192.168.1.1 - 192.168.1.126 | 126 | |
| 2 | 192.168.1.128/25 | 192.168.1.129 - 192.168.1.254 | 126 |
Comprehensive Guide to CIDR, Subnet Masks & IP Addressing
01.Understanding CIDR Notation and Prefix Length
Classless Inter-Domain Routing (CIDR) replaced the legacy Class A, B, and C system in 1993 (RFC 1519) to slow the depletion of IPv4 addresses and control routing table explosion.
A CIDR block is expressed as an IP address followed by a slash and prefix length (e.g., 192.168.1.0/24). The number following the slash denotes the count of contiguous leading '1' bits in the 32-bit subnet mask:
- /24: 24 network bits, 8 host bits → 256 total addresses (254 usable).
- /16: 16 network bits, 16 host bits → 65,536 addresses (65,534 usable).
- /8: 8 network bits, 24 host bits → 16,777,216 addresses.
02.Why Subtract 2 from Total Hosts? (Network vs Broadcast)
In traditional standard IPv4 subnets (prefix length ≤ 30):
- The Network Address: The very first address where all host bits are 0 (e.g.,
192.168.1.0). Used by routers to identify the network segment. - The Broadcast Address: The final address where all host bits are 1 (e.g.,
192.168.1.255). Packets sent to this destination are delivered to every host on that subnet.
Hence, usable hosts equals 2^(32 - prefix) - 2.
Special Exceptions: RFC 3021 defines /31 subnets for point-to-point links (both IPs usable, no broadcast). A /32 represents a single host loopback.
03.RFC 1918 Private IP Address Spaces
To preserve globally routable address space, three blocks are reserved exclusively for private internal networks:
| Block | CIDR | Hosts |
|---|---|---|
| 10.0.0.0/8 | 10.0.0.0 – 10.255.255.255 | 16,777,214 |
| 172.16.0.0/12 | 172.16.0.0 – 172.31.255.255 | 1,048,574 |
| 192.168.0.0/16 | 192.168.0.0 – 192.168.255.255 | 65,534 |
Additionally, 100.64.0.0/10 is reserved under RFC 6598 for Carrier-Grade NAT (CGNAT) by telecom operators.
04.Wildcard Masks vs Subnet Masks
While standard subnet masks indicate which bits represent the network (255.255.255.0), a wildcard mask is an inverted mask (0.0.0.255) primarily utilized in:
- Cisco IOS Access Control Lists (ACLs) to match address ranges.
- OSPF and EIGRP routing protocol network statements.
Formula: Wildcard = 255.255.255.255 - SubnetMask. A 0 bit means "match exactly"; a 1 bit means "ignore this bit".