Subnet & CIDR Calculator (IPv4/IPv6) — Network, Wildcard, VLSM

🧭 Overview

This IP / Subnet / CIDR Calculator instantly converts between CIDR notation and dotted subnet masks, and computes the network address, broadcast address (IPv4), first/last usable IP, usable host count, and wildcard mask for ACLs. It supports IPv4 and IPv6, plus a practical VLSM (Variable Length Subnet Masking) helper for planning subnets.

Typical searches this tool answers: subnet calculator, CIDR calculator, CIDR to subnet mask, wildcard mask calculator, IPv6 prefix calculator, VLSM calculator, how many hosts in /26, what is my network address.

⚡ Quick Start

1. Paste an IP as address/prefix (e.g., 192.168.1.10/24 or 2001:db8::/48) or enter the IP and provide a dotted mask (255.255.255.0) or /prefix in the mask box.
2. Click Calculate / Update. The result table shows network, broadcast, usable range, wildcard, and host counts.
3. Open Advanced → VLSM helper to split a parent network into equal subnets or per-subnet host requirements.
4. Use Copy, Export to TXT, Print/PDF, or Share Link to save or share.

⚙️ How This Tool Works

• IPv4 math: Convert IP to a 32-bit integer. Subnet mask from prefix: (0xFFFFFFFF << (32 − p)) & 0xFFFFFFFF. Network = ip & mask. Broadcast = network | (~mask).
• Usable range (IPv4): For /0…/30, usable hosts = 2^(32 − p) − 2, first IP = network + 1, last IP = broadcast − 1. Special cases: /31 has 2 usable (point-to-point, no broadcast); /32 has 1 host.
• Wildcard (IPv4): The ACL wildcard is the bitwise NOT of the mask; e.g., 255.255.255.0 → 0.0.0.255.
• IPv6: No broadcast/usable concept. We show the prefix start and prefix end by setting host bits to 0/1 using 128-bit arithmetic (BigInt).
• VLSM (IPv4): For equal splits, we add bits to the parent prefix. For a host list, we pick the smallest prefix that fits each request (largest first), align to block boundaries, and step through the parent space.

🧮 Subnet Formulas (Reference)

IPv4 mask from prefix p (0 ≤ p ≤ 32):
  mask = (0xFFFFFFFF << (32 - p)) & 0xFFFFFFFF

Network & broadcast (IPv4):
  network   = ip & mask
  broadcast = network | (~mask)

Wildcard (IPv4 ACL):
  wildcard = (~mask) & 0xFFFFFFFF

Usable host count (IPv4):
  if p ≤ 30: hosts = 2^(32 - p) - 2
  if p = 31: hosts = 2       (point-to-point)
  if p = 32: hosts = 1       (single host)

VLSM: smallest prefix for N hosts (IPv4):
  if N = 1 → /32
  if N = 2 → /31
  else     → p = 32 - ceil(log2(N + 2))   // +2 for net/bcast

📒 IPv4 CIDR Cheat Sheet

*Usable hosts follow traditional rules; /31 has 2 usable (RFC 3021), /32 is a single host.

🧬 IPv6 Prefix Guide (No Broadcast)

IPv6 uses 128-bit addresses and has no broadcast. Most LANs allocate /64; organizations commonly receive /48 or /56.

Special ranges: Link-local fe80::/10, Unique-Local fc00::/7 (commonly fd00::/8), Documentation 2001:db8::/32.

🧪 Worked Examples

1) IPv4 with CIDR

Input: 192.168.1.10/24
Mask:  255.255.255.0
Net:   192.168.1.0
Bcast: 192.168.1.255
First: 192.168.1.1
Last:  192.168.1.254
Hosts: 254
Wildcard: 0.0.0.255

2) IPv4 with dotted mask

Input: 172.16.5.20  Mask: 255.255.255.224 (/27)
Net:   172.16.5.0
Bcast: 172.16.5.31
Usable: 172.16.5.1 – 172.16.5.30 (30 hosts)
Wildcard: 0.0.0.31

3) IPv4 /31 point-to-point

Input: 10.0.0.0/31
Usable IPs: 10.0.0.0 and 10.0.0.1 (2 usable, no broadcast)

4) VLSM hosts list

Parent: 192.168.10.0/24
Requests (hosts): 60, 30, 10

Assigned:
  60 → 192.168.10.0/26     usable 62
  30 → 192.168.10.64/27    usable 30
  10 → 192.168.10.96/28    usable 14
Remaining space continues from 192.168.10.112/28 ...

5) IPv6 prefix preview

Input: 2001:db8::/48
Start: 2001:db8::               End: 2001:db8:0:ffff:ffff:ffff:ffff:ffff
(No broadcast in IPv6)

🧩 Popular Use Cases

• Designing VLANs and LAN segments with the right host capacity.
• Allocating router point-to-point links with /31 (IPv4) or /127 (IPv6).
• Generating ACL wildcard masks for firewalls and routers.
• Planning IPv6 addressing at /48, /56, and /64 boundaries.
• Teaching subnetting, CIDR aggregation, and VLSM to students.

❓ FAQs

🧯 Troubleshooting

• “Invalid IPv4 address” — check octets are 0–255 with no leading/trailing spaces.
• “Invalid dotted subnet mask” — mask must be contiguous 1-bits (e.g., 255.255.0.255 is invalid).
• Overlapping VLSM outputs — ensure the parent prefix is large enough; the tool allocates largest requests first and aligns to block boundaries.
• IPv6 needs a prefix — include /64, /56, etc. Dotted masks don’t apply to IPv6.

📚 Glossary

• CIDR — Classless Inter-Domain Routing; address/prefix notation.
• Subnet mask — a dotted decimal mask for IPv4 (e.g., 255.255.255.0).
• Wildcard mask — inverse of subnet mask, used by ACLs.
• Network address — all host bits zero.
• Broadcast address (IPv4) — all host bits one (not used for /31).
• VLSM — Variable Length Subnet Masking; allocating different-sized subnets from one parent.

📚 Data Sources & Attribution

• RFC 4632 — Classless Inter-Domain Routing (CIDR) for IPv4
• RFC 3021 — Using 31-Bit Prefixes on IPv4 Point-to-Point Links
• RFC 4291 — IPv6 Addressing Architecture
• RFC 4193 — Unique Local IPv6 Unicast Addresses
• RFC 6890 — Special-Purpose Address Registries
• RFC 1918 — Address Allocation for Private Internets