More about CIDR blocks and explanation of IP address portions

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✅ 1. What is a CIDR block?

A CIDR block is a way to represent an IP address range using this format:

<base IP>/<prefix length>

Example:

10.0.0.0/16

It tells you two things:

1) Network portion — the part that identifies the network

2) Host portion — the part used for individual devices (hosts) inside that network

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🎯 2. Network Portion vs Host Portion

Let’s take this example:

10.0.0.0/16

Binary representation of the IP:

00001010.00000000.00000000.00000000

Prefix “/16” means:

• First 16 bits → network portion

• Last 16 bits → host portion

So:

Portion	What it means	Controls
Network portion	Fixed	Defines your “overall network” or VPC size
Host portion	Variable	Defines number of usable IP addresses inside the network

Visual:

10 . 0 . | 0 . 0
         ↑ host portion (16 bits)

So what does this allow?

10.0.0.0 → 10.0.255.255

Meaning:

• All IPs from 10.0.0.0 to 10.0.255.255

• Total = 65,536 IPs (but AWS reserves 5 per subnet)

Why we care:

• Network portion determines the size of the network.

• Host portion determines how many subnets and hosts you can carve out.

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🧩 3. Why do VPC examples use 10.0.x.x/16?

Because 10.x.x.x is part of the RFC 1918 private IP ranges, which are reserved for private networks (LANs, VPCs):

10.0.0.0       – 10.255.255.255      (/8)
172.16.0.0     – 172.31.255.255      (/12)
192.168.0.0    – 192.168.255.255     (/16)

These addresses:

• do not exist on the public internet

• are not routable globally

• are safe for internal use

• are industry standard for private networks and cloud VPCs

This is why AWS, GCP, Azure examples use:

• 10.0.0.0/16

• 172.31.0.0/16

• 192.168.0.0/24

Using 10.0.0.0/16 is just a convention, but a widely adopted one.

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📌 Why specifically 10.0.0.0/16 for VPC labs?

Because it is:

• easy to understand

• easy to subdivide e.g.:

Subnet	CIDR	Type
10.0.1.0/24	Public	Subnet A
10.0.2.0/24	Public	Subnet B
10.0.10.0/24	Private	Subnet A
10.0.11.0/24	Private	Subnet B

• large enough (65k addresses) but not massively oversized

• unlikely to overlap with corporate networks (a real concern in enterprise AWS)

Enterprises often intentionally avoid overlapping IPs to avoid VPN conflicts.

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👍 Is using 10.0.0.0/16 general practice?

Yes. Very common. But NOT a rule.

Common patterns in real companies:

• Small VPC: 10.0.0.0/16

• Medium: 10.0.0.0/12

• Large: 10.0.0.0/8 (rare, enterprise only)

• Multi-environment split:

  • prod: 10.10.0.0/16

  • stage: 10.20.0.0/16

  • dev: 10.30.0.0/16

Cloud providers also pick defaults:

• AWS default VPC: 172.31.0.0/16

• GCP default network: 10.128.0.0/9

All chosen from private IP ranges.

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🔥 Putting it all simply:

Network portion (/16)

• Defines your entire VPC size

• Doesn’t change within the VPC

Host portion

• Used to carve subnets inside the VPC

• Public vs Private subnets take slices of this portion

Why 10.0.x.x?

Because:

• It belongs to the private IP ranges recommended by IPv4 standards

• It’s clean, predictable, and large

• AWS and cloud tutorials commonly use it

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✅ What the Network Portion Actually Does

The network portion of an IP address does more than just identify a VPC or determine how many IPs you get. Its real purpose is deeper and fundamental to how devices communicate.

Let’s break it down clearly.

✔ 1. Identifies the network

It tells routers:

“This IP belongs to this specific network.”

Example: 10.0.0.0/16 → all IPs from 10.0.x.x belong to the same logical network.

But that’s just the first job.

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✔ 2. Tells routers whether two devices are on the same network or different networks

This is the MOST important function.

A device will ask:

• “Is the destination IP in my network portion?” → If yes, send the packet directly using Layer 2 (no router).

• “If not, send the packet to the default gateway (router).”

Example: Device = 10.0.1.50 Subnet = 10.0.1.0/24 Destination = 10.0.1.80

→ Same subnet → communicate directly (ARP + Layer 2 frames)

Destination = 10.0.5.20 → Different subnet → send to router

This logic applies both in AWS and in traditional networking.

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✔ 3. Enables segmentation and isolation

By choosing different network portions, you can create:

• public subnets

• private subnets

• restricted zones

• DMZs

• workload isolation (e.g., databases separate from app servers)

In AWS:

• Public subnet: route to Internet Gateway

• Private subnet: route or to NAT, VPC endpoint, etc.

The segmentation works because the network portions differ.

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✔ 4. Determines the size of the network (host portion)

Yes—changing the prefix length changes how many IPs you get.

• /16 → ~65k addresses

• /20 → 4096

• /24 → 256

But this is almost a side effect of how subnetting works — not the only purpose.

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✔ 5. Controls routing at every hop

Routers match traffic to routing table entries based on the network portion.

Example in AWS inside a subnet:

Destination      Target
10.0.0.0/16      local
0.0.0.0/0        igw-xxxx

The “local” entry exists because of the network portion.

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📌 So the network portion does all of this:

Role	Description
Identifies your network	Defines “this group of IPs belongs together.”
Determines same vs different subnet	Decides whether traffic uses Layer 2 or goes to a router.
Enables segmentation	Lets you carve public/private/subnets logically.
Supports routing decisions	Used by routers to send traffic through the right path.
Defines network size	Determines how many host IPs exist (host portion bits).

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🔥 VPC-specific context

In AWS, the network portion:

• Defines the entire VPC (10.0.0.0/16)

• Ensures subnets fit inside it (10.0.1.0/24, 10.0.2.0/24, etc.)

• Enables AWS built-in “local” routing

• Helps determine overlap with other VPCs when using peering/TGW/Direct Connect

So its purpose goes far beyond just “identifying and counting addresses.”

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🎯

The network portion is critical for:

• Deciding how devices communicate

• Routing

• Isolation

• Segmentation

• Determining network size

• Defining logical boundaries

The host portion is simply "everything that can be assigned an IP address" inside that boundary.

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