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IP Subnet Calculator

IPv4 Calculator
IPv6 Calculator

Student & Admin Resource Guide

Networking can be complex. To help you master subnetting and IP addressing, we have compiled cheat sheets and explanations of core concepts below.

Subnetting Cheat Sheet (IPv4)

This quick reference shows the most common CIDR notations used in local networks.

CIDR Subnet Mask Total IPs Usable Hosts
/32255.255.255.25511 (Host Route)
/30255.255.255.25242 (P2P Links)
/29255.255.255.24886
/28255.255.255.2401614
/27255.255.255.2243230
/26255.255.255.1926462
/25255.255.255.128128126
/24255.255.255.0256254 (Standard LAN)

Public vs. Private IP Addresses

Not all IP addresses can be used on the public internet. The IETF reserved specific ranges for private use within homes and businesses (defined in RFC 1918). These addresses are non-routable on the open web, which preserves the limited pool of IPv4 addresses.

Class RFC 1918 Range Typical Usage
Class A 10.0.0.0 - 10.255.255.255 Large Enterprise Networks
Class B 172.16.0.0 - 172.31.255.255 Universities, AWS/Cloud VPCs
Class C 192.168.0.0 - 192.168.255.255 Home Routers (e.g., 192.168.1.1)

Understanding IPv4 Address Classes

Before the introduction of CIDR (Classless Inter-Domain Routing), IPv4 addresses were divided into five standard classes (A through E). While modern networks rely on CIDR, understanding these legacy classes is essential for network administration and certification exams.

Class Range Default Subnet Usage & Examples
Class A 1.0.0.0 to 126.0.0.0 255.0.0.0 (/8) Used by massive organizations (governments, large ISPs). Supports 16 million hosts.
Example: 10.0.0.1 (Private Network)
Class B 128.0.0.0 to 191.255.0.0 255.255.0.0 (/16) Medium-sized networks like universities and large corporations. Supports 65,000 hosts.
Example: 172.16.0.5
Class C 192.0.0.0 to 223.255.255.0 255.255.255.0 (/24) Small networks, home LANs, and small businesses. Supports 254 hosts.
Example: 192.168.1.1 (Home Router)
Class D 224.0.0.0 to 239.255.255.255 N/A Reserved for Multicasting (streaming video to multiple users).
Class E 240.0.0.0 to 255.255.255.254 N/A Reserved for experimental and future use (Research).

Note: The 127.x.x.x range is excluded from Class A as it is reserved for loopback testing (e.g., localhost 127.0.0.1).

IPv6 Prefix Length Explained

IPv6 abandons the concept of "Subnet Masks" in favor of "Prefix Lengths." This simple number (preceded by a slash, e.g., /64) indicates how many bits of the address are fixed to identify the network.

How to use Prefix Lengths

When configuring an IPv6 network, you choose a prefix length to define the size of your subnet. Smaller numbers mean larger networks.

  • /32 (Provider Assignment): Typically assigned by an Internet Registry to a large ISP. It contains billions of subnets.
  • /48 (Site Assignment): The standard block given to a single organization or company. It allows the company to create 65,536 subnets.
  • /56 (Small Site): Often assigned to small businesses or home networks by ISPs.
  • /64 (Standard Subnet): The industry standard for a single LAN (Local Area Network) or VLAN. Always use /64 for local networks to ensure compatibility with features like SLAAC (Stateless Address Auto-Configuration).
  • /128 (Host Route): Represents a single specific device, similar to a /32 in IPv4.

Example Usage

If your ISP gives you the prefix 2001:db8:abcd::/48, you can create subnets like:

  • 2001:db8:abcd:0001::/64 (Sales Dept)
  • 2001:db8:abcd:0002::/64 (HR Dept)

Recommended External Resources

For students preparing for certifications like the Cisco CCNA or CompTIA Network+, we recommend these authoritative sources: