Module 5: Understanding IPv4 Subnetting

Module 4 explained how an Internet Protocol version 4 (IPv4) address and subnet mask let a host decide whether a destination is local. This module looks at the boundary itself: how one address block is divided into smaller networks and how to describe each resulting range.

Subnetting is not primarily a memorization exercise. It is a way to answer practical questions:

• Which addresses belong to this network?
• Which address identifies the network?
• Which address is used for broadcast traffic?
• How many host addresses fit inside the range?
• Is a destination local, or must traffic use a router?

In This Module

• Why networks are divided into subnets
• How a longer prefix creates a smaller address range
• How to find the network, broadcast, and usable host addresses
• How to choose a prefix for the number of addresses a network needs

Why Subnetting Exists

A subnet is a range of Internet Protocol (IP) addresses that share a network prefix.

Suppose an organization chooses 192.168.10.0/24 for an internal network. It could use the entire range as one network, or divide it into smaller networks for different locations, device groups, or routing boundaries.

One /24 network
        |
        +-- two /25 networks
        |
        +-- four /26 networks

Dividing the range does not create more addresses. It changes how the existing addresses are grouped. Each smaller subnet receives its own network address, broadcast address, and usable host range.

Longer Prefix, Smaller Subnet

The prefix length states how many of the 32 address bits belong to the network.

• A /24 uses 24 network bits and leaves 8 host bits.
• A /25 uses 25 network bits and leaves 7 host bits.
• A /26 uses 26 network bits and leaves 6 host bits.

Each bit moved from the host portion to the network portion doubles the number of subnets and halves the number of addresses in each subnet.

Prefix	Equal ranges inside a /24	Addresses per range	Usually usable by hosts
/24	1	256	254
/25	2	128	126
/26	4	64	62
/27	8	32	30
/28	16	16	14
/29	32	8	6
/30	64	4	2

For ordinary IPv4 subnets through /30:

Host bits = 32 - prefix length
Total addresses = 2^(host bits)
Usable host addresses = total addresses - 2

The subtraction reserves one address to identify the network and one for broadcast traffic.

Network, Broadcast, and Usable Hosts

An ordinary IPv4 subnet contains:

• A network address, which identifies the subnet
• A broadcast address, which reaches all IPv4 hosts on that subnet
• The usable host addresses between them

For 192.168.10.0/24:

Purpose	Address
Network address	192.168.10.0
First usable host	192.168.10.1
Last usable host	192.168.10.254
Broadcast address	192.168.10.255

The first and last addresses have protocol roles, so ordinary hosts use the addresses between them.

Dividing a /24 into /25 Networks

Changing /24 to /25 uses one more network bit. One additional bit has two possible values, so the /24 becomes two equal ranges:

Subnet	Complete range	Usable host range
192.168.10.0/25	.0 through .127	.1 through .126
192.168.10.128/25	.128 through .255	.129 through .254

Each range contains 128 total addresses. The first address is the network address, and the last is the broadcast address.

Dividing a /24 into /26 Networks

Changing /24 to /26 uses two more network bits. Two bits have four possible values, so the /24 becomes four equal ranges:

Subnet	Complete range	Usable host range
192.168.10.0/26	.0 through .63	.1 through .62
192.168.10.64/26	.64 through .127	.65 through .126
192.168.10.128/26	.128 through .191	.129 through .190
192.168.10.192/26	.192 through .255	.193 through .254

Each /26 contains 64 total addresses and normally provides 62 usable host addresses.

The boundaries occur every 64 addresses:

0, 64, 128, 192

That distance between consecutive network addresses is called the block size.

The Repeating Block-Size Pattern

When the subnet boundary is in the final octet, subtract that octet of the mask from 256:

Block size = 256 - mask value

For /26:

Subnet mask: 255.255.255.192
Block size:  256 - 192 = 64

The same pattern repeats as the prefix grows:

Prefix	Final mask octet	Block size
/24	0	256
/25	128	128
/26	192	64
/27	224	32
/28	240	16
/29	248	8
/30	252	4

For prefixes whose boundary falls in an earlier octet, the same pattern applies in that changing octet.

Find the Subnet Containing an Address

Consider the host:

192.168.10.77/26

A /26 has a block size of 64. Its network boundaries in the final octet are:

0, 64, 128, 192

The host value 77 falls between 64 and 127, so:

Purpose	Address
Network address	192.168.10.64
First usable host	192.168.10.65
Host being examined	192.168.10.77
Last usable host	192.168.10.126
Broadcast address	192.168.10.127

A destination from .65 through .126 is local to this host. A destination outside the /26 is remote, even when the first three octets look the same.

For example, 192.168.10.150 would have been local in the larger /24, but it is remote from 192.168.10.77/26.

Choosing a Prefix

A network needs enough host bits for its devices plus the network and broadcast addresses.

Suppose a network needs at least 50 usable host addresses:

5 host bits -> 2^5 = 32 total addresses -> not enough
6 host bits -> 2^6 = 64 total addresses -> 62 usable

Six host bits leave 26 network bits:

32 total bits - 6 host bits = /26

A /26 is therefore the smallest ordinary subnet that meets the requirement.

Real designs also allow room for growth and may reserve addresses for gateways, infrastructure, or platform-specific use. The smallest mathematical answer is not always the best operational design.

Special Cases: /31 and /32

Useful Context

These prefixes are worth recognizing, but they do not change the ordinary subnet model used above.

• A /31 contains two addresses. On an IPv4 point-to-point link, both can be used because that link does not need ordinary network and broadcast addresses.
• A /32 identifies one exact IPv4 address. It commonly appears in host routes and firewall rules.

How a Host Applies the Mask

Technical Detail

The block-size method is easier for people to calculate. This section shows the bit operation a host performs underneath it.

A host finds its network address by comparing its address and mask one bit at a time with a bitwise AND operation. A result bit is 1 only when both input bits are 1.

For 192.168.10.77/24:

Address   11000000.10101000.00001010.01001101   192.168.10.77
Mask      11111111.11111111.11111111.00000000   255.255.255.0
Result    11000000.10101000.00001010.00000000   192.168.10.0

Wherever the mask contains a 1, the address bit passes into the result. Wherever the mask contains a 0, the result becomes 0. What remains is the network address.

Optional Lab: Observe a Wrong Mask

This exercise requires the optional NETLAB environment.

On the Windows Virtual Machine (WINCLIENT), record the current address, mask, gateway, and Domain Name System (DNS) settings. Temporarily configure the same values with a broader /16 mask.

Capture with the Wireshark display filter arp, clear the Address Resolution Protocol (ARP) cache with arp -d *, and ping 10.0.30.1. WINCLIENT should send an unanswered ARP request because the wrong /16 makes that address appear local.

Return the adapter to automatic Dynamic Host Configuration Protocol (DHCP) configuration afterward, run ipconfig /renew, and confirm that the original /24 mask returns.

Further Learning

• Request for Comments (RFC) 4632: Classless Inter-Domain Routing explains classless prefixes and address aggregation.
• RFC 3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links defines the /31 exception.
• Ed Harmoush’s Subnetting Mastery series provides a structured video course with explanations and worked examples.
• freeCodeCamp’s Subnet Cheat Sheet is a quick reference for CIDR prefixes, subnet masks, wildcard masks, and address counts.
• After the concepts make sense, SubnetIPv4.com provides randomly generated address, network, host-range, broadcast, and next-subnet problems for optional repetition.

Main Takeaways

• Subnetting divides an address block into smaller networks without creating additional addresses.
• A longer prefix creates more subnets with fewer addresses in each subnet.
• The prefix determines the network, broadcast, and usable host range and therefore which destinations a host considers local.

Continue to Module 6: Routing to see how packets move between the networks that subnetting defines.