Lesson: Subnetting & CIDR

What you'll learn

What a subnet mask is and what the /24 in 10.100.100.0/24 means.
How to split an address into its network part and host part.
Why the network address and broadcast address can't be used for hosts.
How to look at any two addresses and decide whether they're on the same network.

This is the chapter that makes the "local or remote?" decision from Lesson 1 concrete.

The lesson

1. Why subnets exist

You can't put every machine in the world on one flat network — it wouldn't scale, and you'd want to separate groups for security and management. So we carve the address space into subnets: smaller networks, each a contiguous block of addresses. Our lab is one such subnet: 10.100.100.0/24.

2. Network part vs host part

Recall an IPv4 address is 32 bits. A subnet splits those 32 bits into two pieces:

10.100.100.7  /24
└──────┬─────┘ └┬┘
 first 24 bits   last 8 bits
 = NETWORK       = HOST
 (which network) (which machine on it)

The network part is shared by every machine on the subnet — it's the network's "identity".
The host part is unique per machine within that subnet.

3. The mask and the `/prefix`

The subnet mask says where the split is. It's 32 bits: 1s for the network part, 0s for the host part.

/24  =  11111111.11111111.11111111.00000000  =  255.255.255.0
        └────── 24 ones ───────┘└─ 8 zeros ─┘

So these two notations mean exactly the same thing:

CIDR notation: 10.100.100.0/24 ← the modern, compact form (CIDR = Classless Inter-Domain Routing)
Mask notation: network 10.100.100.0, mask 255.255.255.0

The number after the slash = how many leading bits are the network part. Bigger prefix = more network bits = fewer hosts per subnet.

/24 → 8 host bits  → 2^8  = 256 addresses (254 usable)
/25 → 7 host bits  → 2^7  = 128 addresses (126 usable)
/16 → 16 host bits → 2^16 = 65,536 addresses

4. The two addresses you can't assign

In every subnet, two host values are reserved:

10.100.100.0    ← NETWORK address  (host bits all 0) — names the subnet itself
10.100.100.255  ← BROADCAST address (host bits all 1) — "everyone on this subnet"

That's why a /24 has 256 total addresses but only 254 usable for machines (.1 through .254). In our lab, .254 is the Jumpbox and .1/the gateway live at the low end — the usable middle is where VMs sit.

5. The skill: "are these two on the same network?"

This is the practical payoff. To decide if address A can talk to address B directly (local) or must go via the gateway (remote), compare their network parts.

For /24 it's easy because the split lands on a dot — just compare the first three octets:

10.100.100.7   and  10.100.100.50   → first 3 octets match → SAME network → local
10.100.100.7   and  10.100.101.50   → 3rd octet differs    → DIFFERENT network → remote (via gateway)

For non-/24 masks you compare bits, not octets — that's the AND operation:

address AND mask = network address
10.100.100.7   = 00001010.01100100.01100100.00000111
255.255.255.0  = 11111111.11111111.11111111.00000000
AND result     = 00001010.01100100.01100100.00000000 = 10.100.100.0

Do this for both addresses; if the network addresses match, they're on the same subnet.

6. See it on the lab

ip addr show          # look for an address like 10.100.100.X/24 — the /24 IS the prefix
ipcalc 10.100.100.7/24    # prints network, broadcast, usable range (install: apt install ipcalc)

ipcalc is the fastest way to check your reasoning while it's still new.

Dig deeper

PracticalNetworking — Subnetting Mastery (the best free visual series on this): https://www.practicalnetworking.net/stand-alone/subnetting-mastery/
Cloudflare Learning — What is a subnet?: https://www.cloudflare.com/learning/network-layer/what-is-a-subnet/
An online subnet calculator to check your hand calculations: https://www.subnet-calculator.com/

Search terms

subnetting explained for beginners
CIDR notation explained /24 /16
subnet mask 255.255.255.0 meaning
network address vs broadcast address
how to tell if two IP addresses are on the same subnet

Check yourself

In 10.100.100.0/24, how many bits are the network part? How many usable host addresses are there?
What is the subnet mask for a /24, written in dotted-decimal?
Which two addresses in any subnet are reserved, and what is each called?
Are 10.100.100.5 and 10.100.100.200 on the same network? How did you decide?
Are 10.100.100.5 and 10.100.99.200 (both /24) on the same network? What happens when .5 tries to reach .200 here?