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Networking Foundations

Module 4: IPv4 Addressing

An Internet Protocol version 4 (IPv4) address identifies a device, but the address alone is not enough. A subnet mask tells the device which addresses are nearby and which ones must be reached through a router.

That local-or-remote decision is the main idea in this module. Module 5 builds on it by explaining how subnet boundaries and address ranges are calculated.

In This Module

Section titled “In This Module”
  • How an IPv4 address, prefix, and subnet mask describe a network
  • How Classless Inter-Domain Routing (CIDR) notation describes a subnet
  • How a host decides whether to use its default gateway
  • How to recognize private IPv4 addresses and the effects of a wrong mask

Four Octets, Thirty-Two Bits

Section titled “Four Octets, Thirty-Two Bits”

An IPv4 address is 32 bits long. The familiar dotted form splits those bits into four 8-bit groups called octets, written in decimal, the everyday base-10 numbers, because that is easier to read and type than a row of ones and zeros.

192.168.10.77
11000000.10101000.00001010.01001101

Eight bits produce 256 possible combinations, counted from 0 to 255. That is the entire reason an octet never goes higher than 255. An address such as 192.168.10.300 is not valid, because eight bits cannot count that high.

The dots are a display convenience. The 32 bits are continuous, and a subnet boundary does not have to land on a dot.

An Address Needs a Prefix

Section titled “An Address Needs a Prefix”

The address by itself does not say where the network ends and the hosts begin.

The prefix length after the slash tells you how many of those 32 bits identify the network.

192.168.10.77/24

In this example, 24 bits identify the network and the remaining 8 bits identify a host inside it.

The prefix can also be written as a subnet mask:

/24 = 255.255.255.0

These are two ways of saying the same thing.

Local or Remote?

Section titled “Local or Remote?”

Before sending a packet, a host compares the destination with its own network.

Suppose the host is 192.168.10.77/24.

  • 192.168.10.100 is in the same /24, so the host uses the Address Resolution Protocol (ARP) to find that destination’s media access control (MAC) address.
  • 192.168.11.20 is outside the /24, so the host sends the frame to its default gateway.

The destination Internet Protocol (IP) address remains 192.168.11.20. Only the local frame is addressed to the gateway’s MAC address.

Local destination  -> ARP for the destination
Remote destination -> ARP for the default gateway

This is why the subnet mask matters. It tells the host which of those two actions to take.

What the Mask Is Doing

Section titled “What the Mask Is Doing”

A 255 in an octet is 11111111 in binary, so the mask has a bit pattern of its own:

255.255.255.0
11111111.11111111.11111111.00000000

A 1 marks a bit that belongs to the network. A 0 marks a bit left over to identify a host inside that network. Lining the mask up under the address shows the split:

Address   11000000.10101000.00001010.01001101   192.168.10.77
Mask      11111111.11111111.11111111.00000000   255.255.255.0
          |-------- network -------| | host |

Counting the ones gives you the prefix length. Twenty-four ones is a /24.

A /24 as a Starting Point

Section titled “A /24 as a Starting Point”

With a /24, the first 24 bits—the first three octets—identify the network. The final octet identifies an address inside it.

192.168.10.77/24
| network  |host|
192.168.10 . 77

This makes /24 a useful introductory boundary. It describes the 256 addresses whose first three octets are 192.168.10, from 192.168.10.0 through 192.168.10.255.

Module 5 explains the special roles within that range and how longer prefixes such as /25 and /26 divide it into smaller networks.

Private IPv4 Addresses

Section titled “Private IPv4 Addresses”

Three address blocks are reserved for private networks:

Private block Address range
10.0.0.0/8 10.0.0.0 through 10.255.255.255
172.16.0.0/12 172.16.0.0 through 172.31.255.255
192.168.0.0/16 192.168.0.0 through 192.168.255.255

These addresses can be reused in homes, businesses, labs, and cloud networks because they are not routed across the public internet.

Private does not mean secure. It describes how an address is allocated and routed, not who is allowed to reach it. Module 7 covers the network address translation (NAT) commonly used when private hosts access public services.

Why Addresses Come in Ranges

Section titled “Why Addresses Come in Ranges”

IPv4 contains a fixed set of 32-bit addresses. Prefix lengths let registries, providers, and organizations allocate that space in blocks instead of handing out unrelated individual addresses.

The private ranges above are reserved blocks. Loopback, link-local, documentation, and other special-purpose ranges are additional blocks. Subnetting applies the same idea inside a network by dividing an assigned range into smaller ranges.

What a Wrong Mask Does

Section titled “What a Wrong Mask Does”

A wrong mask changes which destinations the host considers local.

  • If the mask is too broad, the host treats some remote destinations as local and sends ARP requests for them instead of using its gateway.
  • If the mask is too narrow, the host treats some local destinations as remote and sends their traffic to its gateway.

When two nearby hosts cannot communicate, compare their IP addresses and subnet masks before investigating the application.

Inspect Your IPv4 Configuration

Section titled “Inspect Your IPv4 Configuration”

Find the IPv4 address, subnet mask, and default gateway on your active interface.

On Windows:

ipconfig /all

On Linux:

ip -4 addr
ip route

On macOS:

ifconfig
route -n get default

Record the active interface’s:

  • IPv4 address
  • Prefix length or subnet mask
  • Default gateway
  • Domain Name System (DNS) server addresses

Do not assume the first displayed adapter is active. Virtualization and virtual private network software can add several interfaces.

Further Learning

Section titled “Further Learning”

Main Takeaways

Section titled “Main Takeaways”
  • A subnet mask separates the network portion of an IPv4 address from the host portion.
  • Classless Inter-Domain Routing (CIDR) notation is a shorter way to write a subnet mask. For example, /24 means 255.255.255.0.
  • A device uses its IPv4 address and subnet mask to determine whether traffic stays on the local network or goes through the default gateway.

Continue to Module 5: Understanding IPv4 Subnetting to see how prefixes divide address space into network, broadcast, and usable host ranges.