VLSM (Variable Length Subnet Masking) allows using different subnet mask lengths within the same network, enabling efficient use of IP address space. Instead of using the same mask everywhere, you can use /24 for large segments, /28 for small ones, and /30 for point-to-point links. This prevents wasting addresses by matching subnet sizes to actual needs.

IP Subnet Calculator Guide

Q: What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation represents an IP address and its associated network mask together in the format address/prefix-length. For example, 192.168.1.0/24 means the first 24 bits define the network and the remaining 8 bits are for host addresses. This replaced the older classful addressing system and allows more flexible allocation of IP address space.

Q: How do you calculate the number of hosts in a subnet?

The number of usable host addresses in a subnet is 2^(32 - prefix_length) - 2. You subtract 2 because the first address is the network address and the last is the broadcast address. For example, a /24 subnet has 2^8 - 2 = 254 usable hosts, a /25 has 126, and a /30 has 2 (commonly used for point-to-point links).

Q: What is a subnet mask?

A subnet mask is a 32-bit number that separates the network portion from the host portion of an IP address. In binary, it consists of consecutive 1s (network bits) followed by consecutive 0s (host bits). For example, 255.255.255.0 (or /24) means the first 24 bits identify the network. ANDing an IP address with its subnet mask gives the network address.

Q: What are private IP address ranges?

RFC 1918 defines three private IP ranges reserved for internal networks: 10.0.0.0/8 (10.0.0.0 - 10.255.255.255, ~16.7 million addresses), 172.16.0.0/12 (172.16.0.0 - 172.31.255.255, ~1 million addresses), and 192.168.0.0/16 (192.168.0.0 - 192.168.255.255, ~65,000 addresses). These addresses are not routable on the public internet and can be reused in any private network.

Q: What is the difference between network address and broadcast address?

The network address is the first address in a subnet (all host bits set to 0) and identifies the subnet itself. The broadcast address is the last address (all host bits set to 1) and is used to send packets to all hosts in the subnet. Neither can be assigned to a host. For example, in 192.168.1.0/24, the network address is 192.168.1.0 and the broadcast address is 192.168.1.255.

1. IPv4 Addressing Basics

Every device on an IP network needs a unique address to communicate. IPv4 (Internet Protocol version 4) uses 32-bit addresses, typically written in dotted-decimal notation as four octets separated by dots: 192.168.1.100. Each octet represents 8 bits and ranges from 0 to 255.

While dotted-decimal is convenient for humans, understanding the underlying binary representation is essential for subnetting. The address 192.168.1.100 in binary is:

192      .168      .1        .100
11000000.10101000.00000001.01100100

IPv4's 32-bit address space provides 2^32 = 4,294,967,296 possible addresses. While this seemed enormous when IPv4 was designed in the 1980s, the explosive growth of the internet has exhausted this space. IPv6 with its 128-bit addresses was developed to solve this, but IPv4 remains dominant through techniques like NAT (Network Address Translation) and CIDR.

An IP address alone does not tell you which part identifies the network and which part identifies the specific host. That separation is defined by the subnet mask.

2. Subnet Masks Explained

A subnet mask is a 32-bit value that divides an IP address into two parts: the network portion (identifying the subnet) and the host portion (identifying the specific device on that subnet). In binary, a subnet mask consists of consecutive 1-bits followed by consecutive 0-bits.

Subnet Mask:  255.255.255.0
Binary:       11111111.11111111.11111111.00000000
              |---- network (24 bits) ---|--host--|

IP Address:   192.168.1.100
Binary:       11000000.10101000.00000001.01100100
              |---- network portion ----|--host--|

Network:      192.168.1.0    (host bits zeroed)
Host part:    0.0.0.100      (network bits zeroed)

To find the network address, perform a bitwise AND between the IP address and the subnet mask:

IP Address:     11000000.10101000.00000001.01100100  (192.168.1.100)
Subnet Mask:    11111111.11111111.11111111.00000000  (255.255.255.0)
                ----------------------------------------
Network Addr:   11000000.10101000.00000001.00000000  (192.168.1.0)

Two devices can communicate directly (without routing) only if they are on the same network -- that is, the network portions of their addresses are identical when masked.

Common Subnet Masks

CIDR    Subnet Mask        Hosts    Common Use
/8      255.0.0.0          16.7M    Large enterprises
/16     255.255.0.0        65,534   Medium networks
/20     255.255.240.0      4,094    Cloud VPC subnets
/24     255.255.255.0      254      Standard LAN segment
/25     255.255.255.128    126      Half a /24
/27     255.255.255.224    30       Small segments
/28     255.255.255.240    14       Very small segments
/30     255.255.255.252    2        Point-to-point links
/32     255.255.255.255    1        Single host route

3. CIDR Notation

CIDR (Classless Inter-Domain Routing, pronounced "cider") notation is the modern standard for expressing IP addresses with their associated network mask. It appends a forward slash and the number of network bits (prefix length) to the IP address: 192.168.1.0/24.

CIDR replaced the older classful addressing system, which rigidly divided addresses into Class A (/8), Class B (/16), and Class C (/24) networks. This rigid classification wasted enormous amounts of address space -- an organization needing 300 addresses would receive a Class B (/16) with 65,534 addresses, wasting over 65,000.

With CIDR, any prefix length from /0 to /32 is valid, allowing precise allocation. An organization needing 300 addresses can receive a /23 (510 usable hosts) instead of a /16.

Classful vs. Classless

Classful (obsolete):
  Class A:  1.0.0.0   - 126.0.0.0     /8   (16.7M hosts each)
  Class B:  128.0.0.0 - 191.255.0.0   /16  (65,534 hosts each)
  Class C:  192.0.0.0 - 223.255.255.0 /24  (254 hosts each)

CIDR (modern): any prefix length
  10.0.0.0/8         (large private network)
  172.16.0.0/12      (medium private network)
  192.168.1.0/24     (single LAN)
  10.0.1.0/28        (14-host segment)
  10.0.0.0/30        (point-to-point link)

4. Calculating Subnet Details

Given an IP address and prefix length, you can calculate all the key details of the subnet.

Step-by-Step Example: 192.168.10.75/26

Given:  192.168.10.75/26

1. Subnet Mask (/26 = 26 ones followed by 6 zeros):
   11111111.11111111.11111111.11000000 = 255.255.255.192

2. Network Address (IP AND mask):
   192.168.10.75  = 11000000.10101000.00001010.01001011
   255.255.255.192= 11111111.11111111.11111111.11000000
                  = 11000000.10101000.00001010.01000000
                  = 192.168.10.64

3. Broadcast Address (network address OR inverted mask):
   Inverted mask  = 00000000.00000000.00000000.00111111
   Network OR inv = 11000000.10101000.00001010.01111111
                  = 192.168.10.127

4. Usable Host Range:
   First host:  192.168.10.65  (network + 1)
   Last host:   192.168.10.126 (broadcast - 1)

5. Number of Usable Hosts:
   2^(32-26) - 2 = 2^6 - 2 = 64 - 2 = 62 hosts

Summary:
  Network:    192.168.10.64/26
  Mask:       255.255.255.192
  First Host: 192.168.10.65
  Last Host:  192.168.10.126
  Broadcast:  192.168.10.127
  Hosts:      62

5. Host Calculations

The number of usable host addresses is determined by the number of host bits (bits not used for the network prefix).

Host bits = 32 - prefix_length
Total addresses = 2^host_bits
Usable hosts = 2^host_bits - 2  (subtract network and broadcast)

Prefix  Host Bits  Total  Usable  Typical Use
/24     8          256    254     Standard LAN
/25     7          128    126     Half LAN
/26     6          64     62      Quarter LAN
/27     5          32     30      Small department
/28     4          16     14      Server VLAN
/29     3          8      6       Small server group
/30     2          4      2       Point-to-point link
/31     1          2      2*      Point-to-point (RFC 3021)
/32     0          1      1*      Loopback / host route

* /31 and /32 are special cases without network/broadcast

Subnetting a Network

Subnetting divides a larger network into smaller segments. For example, splitting a /24 into four /26 subnets:

Original: 192.168.1.0/24 (254 hosts)

Split into 4 x /26 (62 hosts each):
  Subnet 1: 192.168.1.0/26    (192.168.1.1   - 192.168.1.62)
  Subnet 2: 192.168.1.64/26   (192.168.1.65  - 192.168.1.126)
  Subnet 3: 192.168.1.128/26  (192.168.1.129 - 192.168.1.190)
  Subnet 4: 192.168.1.192/26  (192.168.1.193 - 192.168.1.254)

Each subnet has its own network and broadcast address.

6. Private IP Address Ranges

RFC 1918 defines three private address ranges that are reserved for internal networks and not routable on the public internet. These ranges can be freely used within any organization.

Range              CIDR           Addresses     Common Use
10.0.0.0/8         10.0.0.0 -     16,777,216    Large enterprises,
                   10.255.255.255               cloud VPCs

172.16.0.0/12      172.16.0.0 -   1,048,576     Medium networks,
                   172.31.255.255               Docker default

192.168.0.0/16     192.168.0.0 -  65,536        Home/small office
                   192.168.255.255              networks

Special Address Ranges

Range              Purpose
127.0.0.0/8        Loopback (localhost)
169.254.0.0/16     Link-local (APIPA, auto-config)
0.0.0.0/0          Default route (all networks)
224.0.0.0/4        Multicast
240.0.0.0/4        Reserved (formerly "Class E")
100.64.0.0/10      Carrier-grade NAT (RFC 6598)

7. Variable Length Subnet Masking (VLSM)

VLSM allows different subnets within the same network to have different prefix lengths, matching subnet sizes to actual requirements and minimizing address waste.

VLSM Example

Suppose you have the network 10.1.0.0/24 (254 hosts) and need to create subnets for:

Engineering department: 100 hosts
Sales department: 50 hosts
Management: 20 hosts
Server VLAN: 10 hosts
Two point-to-point links: 2 hosts each

Without VLSM (fixed /26 = 62 hosts each):
  Need 6 subnets but only have 4 x /26 available.
  Cannot accommodate 100-host engineering subnet.

With VLSM (variable subnet sizes):
  10.1.0.0/25    Engineering  (126 hosts) -- 100 needed
  10.1.0.128/26  Sales        (62 hosts)  -- 50 needed
  10.1.0.192/27  Management   (30 hosts)  -- 20 needed
  10.1.0.224/28  Servers      (14 hosts)  -- 10 needed
  10.1.0.240/30  Link 1       (2 hosts)
  10.1.0.244/30  Link 2       (2 hosts)
  10.1.0.248/29  Future use   (6 hosts)

Total: 254 addresses efficiently allocated across 6 subnets.

The key to VLSM is allocating the largest subnets first and working down to the smallest. This ensures address ranges do not overlap and space is used efficiently.

8. Network Design Examples

Small Office Network

Network: 192.168.1.0/24

Subnets:
  192.168.1.0/26    LAN (62 hosts: workstations, printers)
  192.168.1.64/26   WiFi (62 hosts: laptops, phones)
  192.168.1.128/28  Servers (14 hosts)
  192.168.1.144/28  VoIP phones (14 hosts)
  192.168.1.160/28  Guest WiFi (14 hosts)
  192.168.1.252/30  Router uplink

Key addresses:
  Default gateway:  192.168.1.1 (router LAN interface)
  DNS server:       192.168.1.129
  DHCP range:       192.168.1.10 - 192.168.1.60

Multi-Site Enterprise

Network: 10.0.0.0/8

Site allocation:
  10.1.0.0/16   Headquarters (65,534 hosts)
  10.2.0.0/16   Branch Office 1
  10.3.0.0/16   Branch Office 2
  10.10.0.0/16  Data Center 1
  10.11.0.0/16  Data Center 2
  10.100.0.0/16 VPN clients

HQ breakdown (10.1.0.0/16):
  10.1.1.0/24   Floor 1 LAN
  10.1.2.0/24   Floor 2 LAN
  10.1.3.0/24   Floor 3 LAN
  10.1.10.0/24  Server VLAN
  10.1.20.0/24  Management VLAN
  10.1.100.0/24 WiFi VLAN

9. Subnetting in Cloud Environments

Cloud providers like AWS, Azure, and GCP use subnetting extensively for Virtual Private Clouds (VPCs). Understanding subnetting is essential for cloud network architecture.

AWS VPC Subnetting

VPC CIDR: 10.0.0.0/16 (65,536 addresses)

Availability Zone A:
  10.0.1.0/24   Public subnet  (web servers, load balancers)
  10.0.11.0/24  Private subnet (application servers)
  10.0.21.0/24  Data subnet    (RDS, ElastiCache)

Availability Zone B:
  10.0.2.0/24   Public subnet
  10.0.12.0/24  Private subnet
  10.0.22.0/24  Data subnet

Availability Zone C:
  10.0.3.0/24   Public subnet
  10.0.13.0/24  Private subnet
  10.0.23.0/24  Data subnet

Note: AWS reserves 5 IPs per subnet (first 4 + last 1)
  /24 subnet = 256 - 5 = 251 usable addresses in AWS

Kubernetes Pod Networks

# Kubernetes requires large CIDR blocks for pod IPs
# Each node gets a /24 by default (254 pods per node)

Cluster pod CIDR:     10.244.0.0/16  (65,536 pod IPs)
Cluster service CIDR: 10.96.0.0/12   (service virtual IPs)
Node CIDR mask:       /24            (per-node pod range)

Node 1 pods: 10.244.0.0/24
Node 2 pods: 10.244.1.0/24
Node 3 pods: 10.244.2.0/24
...

10. Using Our Free IP Subnet Calculator Tool

Our IP Subnet Calculator instantly computes all subnet details from any IP address and prefix length or subnet mask. Enter an address like 192.168.1.100/24 and see the network address, broadcast address, usable host range, number of hosts, subnet mask, wildcard mask, and binary representation.

The tool also supports subnet division (splitting a network into smaller subnets), VLSM planning, and provides a reference table of all prefix lengths with their corresponding masks and host counts. Whether you are designing a new network, troubleshooting connectivity, or studying for a networking certification, this calculator handles the math instantly.

Try the IP Subnet Calculator

Frequently Asked Questions

What is CIDR notation?

CIDR notation represents an IP address with its network mask as address/prefix-length (e.g., 192.168.1.0/24). The prefix length indicates how many bits define the network. It replaced classful addressing for more flexible IP allocation.

How do you calculate the number of hosts in a subnet?

Usable hosts = 2^(32 - prefix_length) - 2. Subtract 2 for the network and broadcast addresses. A /24 has 254 usable hosts, a /25 has 126, and a /30 has 2.

What is a subnet mask?

A 32-bit number separating the network portion from the host portion of an IP address. Consecutive 1-bits mark the network, 0-bits mark the host part. For example, 255.255.255.0 (/24) means 24 network bits and 8 host bits.

What are private IP address ranges?

RFC 1918 defines three private ranges: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. These are not routable on the public internet and can be reused freely in any private network.

What is the difference between network address and broadcast address?

The network address (all host bits = 0) identifies the subnet. The broadcast address (all host bits = 1) sends to all hosts on the subnet. Neither can be assigned to a device.

What is VLSM?

Variable Length Subnet Masking allows different prefix lengths within the same network, matching subnet sizes to actual requirements. This prevents wasting addresses by using /24 for large segments and /30 for point-to-point links.

IP Subnet Calculator Guide: CIDR and Subnetting Explained