Classful addresses
An IPv4 address consists of 32 bits. Each string of numbers separated by the period consists of 8 bits, represented by 0 to 255 in numerical forms. Organizations could purchase three classes of IPv4 addresses.
Class A
A Class A IPv4 address has 8 network prefix bits. For example, consider 44.0.0.1, where 44 is the network address and 0.0.1 is the host address.
Class B
A Class B IPv4 address has 16 network prefix bits. For example, consider 128.16.0.2, where 128.16 is the network address and 0.2 is the host address.
Class C
A Class C IPv4 address has 24 network prefix bits. For instance, consider 192.168.1.100, where 192.168.1 is the network address and 100 is the host address.
What are the limitations of classful IP addressing that CIDR overcomes?
Before Classless Inter-Domain Routing (CIDR), IP addresses were classful and created inefficiencies. We discuss some of these shortcomings next.
Inflexible IP addressing
In a classful addressing system, each class supported a fixed number of devices:
- Class A supported 16,777,214 hosts
- Class B supported 65,534 hosts
- Class C supported 254 hosts
The classful arrangement was inefficient when allocating IP addresses and led to a waste of IP address spaces.
For example, an organization with 300 devices couldn’t have used a Class C IP address, which only permitted 254 devices. So, the organization would’ve been forced to apply for a Class B IP address, which provided 65,534 unique host addresses. However, only 300 devices would’ve been connected, which would’ve left 65,234 unused IP address spaces.
Limitations in network design
Classful IPs limited your ability to combine networks as required. For example, these IP addresses belong to different class C networks in the classful architecture:
- 192.168.1.0
- 192.168.0.0
As a network administrator, you couldn’t have combined both networks because the class C subnet mask was fixed as 255.255.255.0.
Conversation
explain more here Limitations in network design Classful IPs limited your ability to combine networks as required. For example, these IP addresses belong to different class C networks in the classful architecture: 192.168.1.0 192.168.0.0 As a network administrator, you couldn’t have combined both networks because the class C subnet mask was fixed as 255.255.255.0.
Sure! Let’s break this down in more detail:
🧠 Background: Classful IP Addressing
In classful IP addressing, IP addresses were divided into fixed classes (A, B, C, etc.), each with a default subnet mask. This system was used before the introduction of CIDR (Classless Inter-Domain Routing).
- Class C networks:
- Range:
192.0.0.0to223.255.255.255 - Default subnet mask:
255.255.255.0(or/24) - That means: 256 IPs per network (254 usable hosts)
- Range:
Each Class C network was expected to cover only 1 subnet with 254 hosts, based on the default subnet mask.
🛑 Problem: Lack of Flexibility
Because classful addressing forced a fixed subnet mask, you couldn’t merge two adjacent class C networks like:
192.168.0.0/24192.168.1.0/24
Even though logically they might represent parts of the same organization, department, or location, the system saw them as completely separate networks.
🔒 Example:
Say you’re managing IPs for two departments in the same building, and you want them on the same subnet. You might think:
“They both use 192.168.x.x. Why not combine them into one network?”
But classful design wouldn’t allow this. 192.168.0.0 and 192.168.1.0 are different class C networks with a fixed mask of /24, and thus cannot be combined.
🔓 Solution (Eventually): CIDR
CIDR (introduced in 1993) allows for variable-length subnet masks, so instead of being stuck with /24 for Class C, you can now use something like:
192.168.0.0/23→ includes both:192.168.0.0 – 192.168.0.255192.168.1.0 – 192.168.1.255
Now you’re using 1 subnet that spans two former class C networks.
✅ Summary
| Feature | Classful | CIDR |
|---|---|---|
| Subnet Mask | Fixed per class (e.g., /24 for Class C) | Flexible (/8 to /30) |
| Can merge adjacent networks? | ❌ No | ✅ Yes |
| Efficient IP use | ❌ Often wasteful | ✅ More efficient |
The main limitation of classful addressing was that it imposed artificial boundaries based on the class system, preventing efficient network design and address allocation.
Let me know if you’d like a diagram to illustrate this visually.
explain more about subnet
Absolutely! Let’s go step by step to help you deeply understand subnets — what they are, why they matter, and how they’re used.
🔹 What Is a Subnet?
A subnet (short for sub -network) is a smaller network segment within a larger IP network.
Think of it like dividing a neighborhood (a network) into individual houses (subnets). Each house still belongs to the neighborhood, but you can manage and control them separately.
🔹 Why Use Subnets?
Subnets help:
- Organize networks logically (e.g., Sales, HR, and Engineering get their own subnets).
- Improve security (e.g., isolate internal systems from public-facing systems).
- Reduce broadcast traffic (only devices in the same subnet hear each other’s broadcast).
- Efficiently allocate IP addresses, especially with CIDR.
🔹 IP Addresses and Subnet Masks
Every IPv4 address has:
- An IP address (e.g.,
192.168.1.10) - A subnet mask (e.g.,
255.255.255.0or/24)
The subnet mask determines:
- Which part of the IP is the network
- Which part is the host
🧠 Example:
IP: 192.168.1.10
Subnet Mask: 255.255.255.0 (or /24)
Convert to binary:
- The first 24 bits (the ones) = network part →
192.168.1.0 - The last 8 bits (the zeros) = host part → possible values:
1to254
So, this subnet (192.168.1.0/24) can have 256 IPs total, but only 254 usable:
192.168.1.0= Network address192.168.1.255= Broadcast address- Usable range:
192.168.1.1–192.168.1.254
🔹 Subnet Mask and CIDR Notation
You’ll often see subnets written like:
/24→255.255.255.0/25→255.255.255.128/26→255.255.255.192
Each higher number (e.g., /25) means more bits for network, and fewer for hosts.
| CIDR | Subnet Mask | Hosts per Subnet |
|---|---|---|
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /26 | 255.255.255.192 | 62 |
| /27 | 255.255.255.224 | 30 |
| /30 | 255.255.255.252 | 2 |
🔧 Real-Life Use Case
Let’s say you’re a sysadmin for a small company:
- You have
192.168.1.0/24available. - You want 3 departments:
- Engineering (100 PCs)
- HR (50 PCs)
- Finance (30 PCs)
Instead of giving each department a whole /24, you subnet the network:
- Engineering →
192.168.1.0/25→ 126 hosts - HR →
192.168.1.128/26→ 62 hosts - Finance →
192.168.1.192/27→ 30 hosts
No wasted IPs, and each department is isolated at layer 3.
🔹 Summary
| Concept | Description |
|---|---|
| Subnet | A smaller part of a larger IP network |
| Subnet mask | Defines which portion of IP is network vs host |
| CIDR notation | A shorthand (e.g., /24) for subnet masks |
| Why subnets? | Organization, security, reduced traffic, efficient IP use |
| Subnetting | Dividing a larger block into multiple logical networks |
Would you like me to show you:
- A subnetting diagram?
- How to calculate subnets manually?
- Or a CIDR calculator tool or example?
Let me know how deep you want to go.
how to check my machine subnet with ifconfig
You can easily check your machine’s subnet information using the ifconfig command on Linux or macOS.
Here’s how to do it in detail:
🧰 1. Run ifconfig
Open your terminal and type:
bashifconfigYou’ll see output like this (simplified):
🧠 2. Key Parts to Look At
| Field | Meaning |
|---|---|
inet | Your local IP address (e.g., 192.168.1.101) |
netmask | The subnet mask (in hexadecimal on macOS, decimal on Linux) |
broadcast | Broadcast address for the subnet |
🧮 3. Understanding netmask
On macOS:
You might see something like:
Convert 0xffffff00 to decimal:
0xffffff00=255.255.255.0- Which means: /24 subnet
On Linux:
You’ll see it directly as:
So your subnet is:
- Network: Based on IP and netmask. For
192.168.1.101and255.255.255.0, the network is192.168.1.0/24 - Subnet Mask:
255.255.255.0 - Broadcast:
192.168.1.255
📝 Optional: Use ip command (Linux only)
More modern than ifconfig:
Example output:
Here, /24 tells you the subnet directly.
✅ Summary
| What you want | How to find it |
|---|---|
| IP address | inet |
| Subnet mask | netmask field |
| Broadcast address | broadcast field |
| CIDR prefix | Convert netmask to CIDR manually, or use ip addr show |
Let me know if you want a CIDR cheat sheet, or want to calculate your network and broadcast address based on IP + subnet.
0xfffffe00
Great — you’ve found a netmask value of 0xfffffe00, and you want to understand what subnet it represents.
Let’s break it down step by step.
🧠 Step 1: Understand 0xfffffe00
This is a hexadecimal representation of a subnet mask.
We’ll convert this to dotted decimal format, and then determine the CIDR prefix (/XX), which tells us how many bits are used for the network portion.
🔣 Step 2: Convert Hex to Dotted Decimal
Hex: 0xfffffe00
Split into bytes:
ff= 255ff= 255fe= 25400= 0
➡️ Decimal format: 255.255.254.0
🔢 Step 3: Determine CIDR Prefix
Now convert 255.255.254.0 to CIDR:
255=11111111→ 8 bits255=11111111→ 8 bits254=11111110→ 7 bits0=00000000→ 0 bits
Total: 8 + 8 + 7 = 23 bits
➡️ CIDR notation: /23
📦 Step 4: What Does a /23 Subnet Mean?
A /23 subnet:
- Subnet mask:
255.255.254.0 - Number of IP addresses:
2^(32-23) = 512 - Usable host IPs: 510 (excluding network and broadcast)
- Covers 2 consecutive /24s
Example:
If your IP is 192.168.10.5 and your netmask is /23, your network covers:
192.168.10.0→ to192.168.11.255- Usable IPs:
192.168.10.1–192.168.11.254
✅ Summary
| Hex Netmask | Dotted Decimal | CIDR | Hosts |
|---|---|---|---|
0xfffffe00 | 255.255.254.0 | /23 | 510 usable |
This subnet is useful when you need more than 254 IPs, but don’t want to jump to a full /22 (1022 usable).
Let me know if you want a quick conversion table from hex → CIDR or subnet mask → hosts!
inet 10.10.15.15 netmask 0xfffffe00 broadcast 10.10.15.255
Let’s analyze this line from your ifconfig output:
🔍 Breakdown
| Field | Value | Meaning |
|---|---|---|
inet | 10.10.15.15 | Your machine’s local IP address |
netmask | 0xfffffe00 | Your subnet mask, in hexadecimal |
broadcast | 10.10.15.255 | The broadcast address for your subnet |
🧠 Convert the Netmask
0xfffffe00 (hex) → decimal:
ff= 255ff= 255fe= 25400= 0
➡️ Decimal Netmask: 255.255.254.0
➡️ CIDR Notation: /23 (as explained earlier)
🌐 What Network Are You In?
Your IP: 10.10.15.15
Netmask: /23 → range = 512 IPs
A /23 groups two /24 blocks together.
We calculate the network address by zeroing out the host bits.
Step-by-step:
Apply the netmask (bitwise AND):
So:
| Field | Value |
|---|---|
| Network Address | 10.10.14.0 |
| Broadcast Address | 10.10.15.255 |
| CIDR Notation | 10.10.14.0/23 |
| Usable IPs | 10.10.14.1 – 10.10.15.254 |
| Total IPs | 512 (510 usable) |
✅ Final Summary
You’re on a /23 subnet, meaning:
- Your network spans from
10.10.14.0to10.10.15.255 - Your machine’s IP
10.10.15.15is within this range - Your subnet supports 510 usable IPs
- You can communicate directly with any device from 10.10.14.1 to 10.10.15.254
Would you like a visual diagram showing how that /23 range breaks down?
yes
why 192.168.1.0 192.168.0.0 elong to different class C networks
Good question. The reason why 192.168.1.0 and 192.168.0.0 are considered different Class C networks under classful IP addressing comes down to how classful IPs were originally defined.
🧠 Classful IP Addressing Refresher
Classful IPs divide the IPv4 space into 5 classes (A–E). Here’s the relevant part for Class C:
✅ Class C
- Starts at:
192.0.0.0 - Ends at:
223.255.255.255 - Default subnet mask:
255.255.255.0(or/24) - Network bits: First 24 bits are the network part
- Host bits: Last 8 bits are for hosts
That means:
- Each Class C network has 256 addresses (254 usable)
- Example:
192.168.0.0/24is a network192.168.1.0/24is a completely different network
📦 Why 192.168.0.0 ≠ 192.168.1.0
Let’s see them in binary (first 3 octets only):
192.168.0.0→11000000.10101000.00000000.00000000192.168.1.0→11000000.10101000.00000001.00000000
Now apply the Class C default subnet mask:
255.255.255.0→ first 24 bits = network part
So:
192.168.0.0/24= IPs from192.168.0.0–192.168.0.255192.168.1.0/24= IPs from192.168.1.0–192.168.1.255
They cannot communicate directly without routing because they’re different subnets.
🛑 The Limitation
In classful days, subnet masks were fixed by class, so you couldn’t say:
“I want both
192.168.0.0and192.168.1.0in the same network.”
You were forced to treat them as two distinct /24 networks.
✅ Solution: CIDR (Classless Inter-Domain Routing)
With CIDR (post-1993), you can define flexible subnet sizes.
You can now do:
This combines two adjacent /24 networks into one /23.
🔚 Summary
| Item | Classful | CIDR |
|---|---|---|
192.168.0.0 and 192.168.1.0 | Two different networks (/24) | Can be merged as one (/23) |
| Subnet mask | Fixed: 255.255.255.0 | Flexible: /23, /22, etc. |
| Network boundaries | Based on 3rd octet | Based on bit mask |
Let me know if you want a visual comparison of how /24 and /23 cover those ranges.
which class this belog to inet 10.10.15.15 netmask 0xfffffe00 broadcast 10.10.15.255
The IP 10.10.15.15 belongs to Class A in the original classful IP addressing system.
🔍 Let’s Break It Down:
🧠 IP Classes Overview:
| Class | Starting Range | Ending Range | Default Subnet Mask | Bits for Network |
|---|---|---|---|---|
| A | 1.0.0.0 | 126.255.255.255 | 255.0.0.0 (/8) | First 8 bits |
| B | 128.0.0.0 | 191.255.255.255 | 255.255.0.0 (/16) | First 16 bits |
| C | 192.0.0.0 | 223.255.255.255 | 255.255.255.0 (/24) | First 24 bits |
💡 IPs starting with
10.x.x.xfall in the Class A range.
🔢 Your IP Details:
- IP:
10.10.15.15 - First octet:
10 - Since
10is between1and126, this is Class A
Additional Notes:
- Private IP range:
10.0.0.0/8is also a private address space (defined by RFC 1918). - So your IP is both:
- Class A
- Private IP
🧱 What About the Netmask?
Your netmask is 0xfffffe00 → 255.255.254.0 → /23
Even though it’s a Class A IP, you’re not using the default /8 subnet mask. You’re using CIDR to define a smaller subnet (a /23), which is completely valid and common in modern networking.
✅ Final Answer:
| Field | Value |
|---|---|
| IP | 10.10.15.15 |
| Class | Class A |
| Private IP? | ✅ Yes (10.0.0.0/8) |
| Subnet (CIDR) | /23 (255.255.254.0) |
| Classful Subnet? | ❌ Not classful (you’re using CIDR) |
Let me know if you want to dive deeper into the private IP ranges or the differences between classful and CIDR-based routing!
is this uisng CIDR
Yes — your configuration is using CIDR, not classful addressing.
Let’s walk through why.
🧾 Your IP Configuration:
Convert netmask:
0xfffffe00→255.255.254.0→ CIDR notation:/23
✅ Why This Is CIDR (Classless Inter-Domain Routing)
1. Not using the default classful subnet mask
- Class A IPs (
10.x.x.x) had a default mask of/8(255.0.0.0) - You’re using /23, which is not classful
➡️ CIDR allows you to choose subnet sizes freely, regardless of the IP’s original class.
2. CIDR separates the IP world from old “class” boundaries
- In CIDR, the idea of “Class A/B/C” is irrelevant.
- You can subnet any way you want —
/12,/19,/23, etc. - That’s what you’re doing with a /23 netmask.
🧠 Classful vs CIDR — Quick Comparison
| Feature | Classful | CIDR |
|---|---|---|
| Subnet size | Fixed per class | Flexible (/8 to /30) |
10.10.15.15 subnet | /8 only | ✅ /23 (your config) |
| Efficient IP usage | ❌ Often wasteful | ✅ Efficient |
| Still used today? | ❌ Deprecated | ✅ Standard today |
✅ Final Answer:
Yes — your IP setup (10.10.15.15/23) is using CIDR. You’re overriding the classful default (/8) with a custom subnet mask (/23) to define your own network boundary, which is exactly what CIDR was created for.
Let me know if you want a deeper breakdown of how /23 works in this context!