IPv4 Address Structure

The Two-Part Address: Street and House Number

An IP address works much like a postal address. A postal address has two main parts: the street name, which identifies a whole street, and the house number, which pinpoints a specific house on that street.

Similarly, an IPv4 address has two parts:

• The Network Portion: Identifies the specific network a device belongs to (the "street").
• The Host Portion: Identifies the specific device on that network (the "house number").

All devices on the same local network, like the computers, phones, and smart devices in your home, will share the same network portion in their IP addresses. They will each have a unique host portion.

To get a more intuitive feel for this, let's watch a video that uses this exact analogy.

The Subnet Mask: Revealing the Structure

Looking at an IP address alone, there's no way to know where the network portion ends and the host portion begins. Is it after the first number? The second? The third?

This is where the subnet mask comes in. A subnet mask is a 32-bit number that accompanies an IP address. Its sole purpose is to "mask" the IP address to reveal which part is the network and which is the host.

Let's start with a simple rule of thumb.

As the video explained, if your IP address is 192.168.1.100 and your subnet mask is 255.255.255.0:

• The first three 255s in the mask tell us that the first three octets of the IP (192.168.1) are the network portion.
• The 0 in the mask tells us that the last octet (100) is the host portion.

This means any device on this network will have an IP address that starts with 192.168.1.x.

The Binary Truth of the Mask

The 255.0.0.0 logic is a helpful simplification, but the real operation happens at the binary level. This is where your computer science background and our previous lesson on binary conversion come into play.

A subnet mask is a 32-bit binary number consisting of a continuous string of 1s followed by a continuous string of 0s.

• The 1s in the mask correspond to the network bits in the IP address.
• The 0s in the mask correspond to the host bits in the IP address.

Let's watch a short video that visualizes this binary relationship perfectly.

Finding the Network Address with a Logical AND

Computers determine the network address (the "street name" itself, with the host part set to all zeros) by performing a bitwise logical AND operation between the IP address and the subnet mask.

Given your background, you're likely familiar with logical operators. In a bitwise AND:

• 1 AND 1 = 1
• 1 AND 0 = 0
• 0 AND 1 = 0
• 0 AND 0 = 0

When a computer performs this operation on a 32-bit IP address and its 32-bit subnet mask, the result is the 32-bit Network ID.

Let's see an example:

• IP Address: 192.168.1.65 -> 11000000.10101000.00000001.01000001
• Subnet Mask: 255.255.255.0 -> 11111111.11111111.11111111.00000000

Performing a bitwise AND:

  11000000.10101000.00000001.01000001  (IP Address)
& 11111111.11111111.11111111.00000000  (Subnet Mask)
-------------------------------------
  11000000.10101000.00000001.00000000  (Resulting Network ID)

Converting the result back to decimal gives us 192.168.1.0. This is the Network ID, or the address of the network itself.

The following reading explains this formal process in detail.

A quick note on history: this flexible system of using masks is called Classless Inter-Domain Routing (CIDR). It replaced an older, rigid system called Classful Addressing (Class A, B, C) where the network/host split was inflexibly determined by the first few bits of the IP address itself. CIDR and subnet masks give network engineers the power to create networks of almost any size, which is essential for the modern internet.

Conclusion

In this lesson, we dissected the structure of an IPv4 address, revealing the two fundamental components that govern how network traffic is routed.

Key Takeaways:

• An IPv4 address is divided into a network portion (the street) and a host portion (the house number).
• The subnet mask is a required companion to an IP address that defines the boundary between these two portions.
• In binary, the 1s in the subnet mask align with the network bits, and the 0s align with the host bits.
• A computer formally identifies the network address by performing a bitwise logical AND between the IP address and the subnet mask.

Preview of the Next Lesson:

Now that you understand the structure, we can explore its implications. In the next lesson, we will focus on the learning outcome: Explain the role of subnet masks in defining network boundaries. We will delve into how changing the subnet mask carves up the vast IP address space into smaller, manageable networks, a process known as subnetting. We'll also formally introduce the "slash notation" (e.g., /24) you may have seen, which is a shorthand for writing subnet masks.