Subnet Calculator — IPv4 / IPv6 CIDR Subnetting Tool Free
Calculate IPv4/IPv6 subnets, CIDR ranges, network/broadcast addresses, host counts, subnet masks. Browser-only, instant.
About Subnet Calculator
A subnet calculator computes IP-network properties from CIDR notation (e.g. 192.168.1.0/24) — the network address, broadcast address, usable host range, total host count, subnet mask in dotted-decimal, and binary representation. Network engineers and sysadmins use it daily for planning IP allocation, configuring routers, and troubleshooting network issues. The ZTools Subnet Calculator handles both IPv4 (32-bit) and IPv6 (128-bit) addressing, supports VLSM (variable-length subnet mask) calculations, and provides a hosts-per-subnet table for splitting a /24 into smaller subnets.
Use cases
- Network design / VLAN planning. Allocating IP ranges across VLANs, departments, or customers. /24 (256 addresses) is too big for a small office; /28 (16 addresses) is right-sized.
- Cloud VPC subnet planning. AWS / GCP / Azure VPCs need subnet CIDR planning. Avoid overlap with on-prem ranges, account for future growth, leave room for AWS reserved addresses (5 per subnet).
- Firewall rules / ACLs. Allow source 10.0.0.0/16 → expand to range; deny 192.168.5.0/27 → calculate exact addresses covered.
- CCNA / network certification study. Subnetting is a core skill on networking certs (CCNA, CompTIA Network+). Calculator validates manual answers during practice.
How it works
- Enter IP/CIDR. `192.168.1.100/24` or `2001:db8::/32`. Both notations supported.
- Or pick mask. Mask in dotted-decimal (255.255.255.0) auto-converts to /24.
- View results. Network, broadcast, first host, last host, total hosts, mask in multiple notations, binary, hex.
- Subnet division (VLSM). Split a /24 into N smaller subnets; calculator generates the table of resulting CIDR blocks.
- IP-in-subnet check. Test if a specific IP falls within a subnet. Useful for ACL verification.
Examples
Input: 192.168.1.0/24
Output: Network: 192.168.1.0; Broadcast: 192.168.1.255; Hosts: 254; Mask: 255.255.255.0.
Input: 10.0.0.0/16 split into 4 equal subnets
Output: 10.0.0.0/18, 10.0.64.0/18, 10.0.128.0/18, 10.0.192.0/18 — 16,382 hosts each.
Input: 2001:db8::/64
Output: IPv6 /64 = 2^64 addresses (~18 quintillion). Host range covers entire /64; first host 2001:db8::1.
Frequently asked questions
Why does a /24 have 254 hosts and not 256?
Network address (.0) and broadcast address (.255) are reserved. 256 total addresses minus 2 = 254 usable hosts. AWS reserves an additional 3 (so 251 usable in AWS VPCs).
What's VLSM?
Variable-Length Subnet Masking — splitting an IP range into subnets of different sizes. Example: a /24 into a /25 (128 addresses) plus 2 /26s (64 each). Allows efficient address allocation.
Difference between /24 and 255.255.255.0?
Same thing in different notation. /24 means "24 ones in the mask"; 255.255.255.0 in binary is `11111111.11111111.11111111.00000000` = 24 ones.
Can I subnet IPv6?
Yes — IPv6 uses identical CIDR notation. Common practice: allocate /48 to organisations, /64 per LAN.
How does AWS subnet differ?
AWS reserves 5 addresses per subnet (.0, .1, .2, .3, .last). Subtract 5 from total host count for usable AWS VPC subnet capacity.
What's a /31 subnet?
Used for point-to-point links (RFC 3021). Only 2 addresses, both usable as hosts (no broadcast). Common between routers.
Pro tips
- Always reserve 20–30% growth headroom in subnet sizing — bigger now is cheaper than re-numbering later.
- Document your subnet plan (spreadsheet or IPAM tool) before deploying — drift is brutal to fix.
- For AWS VPC, account for the 5 reserved addresses per subnet.
- Use private RFC1918 ranges (10/8, 172.16/12, 192.168/16) for internal — never public space without ownership.
- Avoid 169.254/16 (link-local, used by APIPA) and 224/4 (multicast) for production subnets.
Reviewed by Ahsan Mahmood · Last updated 2026-05-06 · Part of ZTools.
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