The Ultimate Suite of
CIDR Tools
Professional-grade network calculators for subnetting, CIDR planning, and IP addressing. Fast, free, and built by network engineers for maximum precision.
Welcome to CIDRtools.net, your go-to destination for free, client-side network utilities. Whether you are auditing VPC peering rules with our subnet overlap detector, planning a large-scale deployment with the VLSM planner, or simply needing a quick CIDR calculator, our tools are designed to be fast and secure. All calculations happen entirely in your browser—no data ever leaves your machine.
IPv4 Tools
CIDR Calculator
Enter any IPv4 CIDR block and instantly get network address, broadcast, subnet mask, wildcard mask, binary breakdown, and usable host range.
Open CIDR Calc →Subnet Tree Visualizer
Interactively visualize and split IPv4 subnets. Start with a parent block and recursively divide it into smaller subnets to plan your network hierarchy.
Open Tree Visualizer →VLSM Planner
Allocate subnets of varying sizes from a parent block. Enter host requirements and get network/broadcast/range for each — sorted and packed automatically.
Open VLSM Planner →Hierarchical Site Planner
Divide large supernets into isolated site-blocks. Plan regional address space while ensuring summary aggregation and preventing regional collisions.
Open Site Planner →Subnet Overlap Detector
Paste a list of CIDR blocks and instantly detect overlapping or duplicate networks. Essential for VPN and VPC peering rule audits.
Open Overlap Detector →IP Range → CIDR
Convert any arbitrary start–end IP range into the minimal set of CIDR blocks covering it exactly. Essential for firewall migrations and AWS Security Groups.
Open IP Range Converter →Mask Converter
3-way live converter between subnet mask, wildcard mask, and CIDR prefix. Full 33-row reference table. Type in any field — all others sync instantly.
Open Mask Converter →Route Aggregator
Condense thousands of specific IP routes into the fewest possible summary CIDR blocks. Supports huge lists. Great for BGP prefix lists and ACL optimization.
Open Aggregator →BGP Prefix-List Generator
Generate optimized BGP prefix-lists and route-map statements (Cisco, Juniper, FRR) from a list of prefixes using lossless ge/le bounds.
Open Generator →IPv6 Calculator
A precision tool for the 128-bit era. Calculate IPv6 network ranges, expanded/compressed formats, and view binary nibbles. Supports all standard prefix lengths.
Open IPv6 Calc →BGP VPN P2P Generator
Generate point-to-point subnets for route-based IPsec VPNs and BGP peering. Automatically pairs Local and Remote CVI/NDI endpoints for HA clusters.
Open P2P Generator →Utilities
MAC Address Formatter
Convert MAC addresses between Cisco, Linux, and Windows formats. Instant offline OUI lookup to identify hardware vendors automatically.
Open MAC Formatter →MTU Path Discovery
Simulate and calculate effective MTU across multiple hops with varying overheads (IPSec, GRE, QinQ). Identify fragmentation risks before deployment.
Open MTU Calc →DNS Lookup
Fast, clean interface for querying DNS records (A, AAAA, MX, TXT, CNAME, NS, CAA) using Cloudflare's secure DNS-over-HTTPS resolver.
Open DNS Lookup →SPF Generator
Visually build and validate SPF (Sender Policy Framework) records. Includes support for includes, IP ranges, and soft/hard fail mechanisms.
Open SPF Generator →Certificate Decoder
Decode SSL/TLS X.509 certificates locally. Paste PEM or drop CRT/DER files to view SANs and expiration. 100% private.
Open Cert Decoder →Reference
Subnet Cheat Sheets
Instant reference tables for IPv4 prefixes, IPv6 allocation sizes, RFC 1918 private ranges, and wildcard masks.
Subnet Cheat Sheets →DNS Record Cheat Sheet
Quick reference guide for common DNS record types (A, MX, TXT, SRV, CAA) with examples and usage descriptions.
DNS Cheat Sheet →IPv4 /31 & /32 Subnets
Understand two-address point-to-point links, single-host routes, address savings, alignment, and deployment checks.
Read the Guide →IPv4 Subnetting Practice
Use a repeatable host-count and boundary-check method to split a /24 into valid subnets without guessing.
Read the Guide →VLSM Subnetting Examples
Follow complete office and multi-zone cloud allocations with capacity planning, valid boundaries, and growth reserves.
Read the Guide →Subnet Overlap & Aggregation
Resolve overlapping VPN and cloud ranges, then learn when contiguous routes can—and cannot—be summarized safely.
Read the Guide →IPv6 /64 Planning & Migration
Allocate site and LAN prefixes from a /48, then introduce IPv6 with a measured dual-stack rollout and acceptance checks.
Read the Guide →Tunnel MTU & TCP MSS
Calculate packet sizes for IPsec, GRE, WireGuard, AWS, and Azure tunnels, then troubleshoot PMTUD black holes.
Read the Guide →DNS & SPF Troubleshooting
Inspect TXT records, combine legitimate senders into one SPF policy, and avoid lookup-budget failures.
Read the Guide →BGP Prefix-List Examples
Compare exact and bounded prefix filters for Cisco IOS, Juniper Junos, and FRRouting with a lossless worked example.
Read the Guide →Common Questions
What is a CIDR tool?
A CIDR tool helps network engineers calculate IP address ranges, subnet masks, and broadcast addresses from a CIDR notation (like 10.0.0.0/24). It simplifies network planning and prevents allocation errors.
Is my data secure?
Yes. CIDRtools.net is a "client-side" application. This means all calculations happen locally in your browser. We never see your IP plans, certificates, or DNS queries.
Does it support IPv6?
Absolutely. We provide a dedicated IPv6 calculator and reference sheets for modern 128-bit network planning.