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Subnetting & CIDR

beginner subnetting cidr subnet-mask ipv4 networking

Subnetting is the act of splitting a big IP block into smaller networks so we can organize them, secure them, and route between them efficiently. CIDR (Classless Inter-Domain Routing) is the modern notation we use for it.

In simple language: take an IP range and slice it up. CIDR tells us “how much of the address is the network, and how much is the host.”

The /N Notation

192.168.1.0/24 means:

  • The first 24 bits identify the network.
  • The remaining 32 - 24 = 8 bits identify the host within that network.
  • That gives us 2^8 = 256 addresses (254 usable — minus the network and broadcast).
192.168.1.0/24
|________| |__|
network    host bits (8)

Subnet Mask

The subnet mask is the binary version of the prefix length:

/24  ->  255.255.255.0     ->  11111111.11111111.11111111.00000000
/16  ->  255.255.0.0       ->  11111111.11111111.00000000.00000000
/8   ->  255.0.0.0         ->  11111111.00000000.00000000.00000000
/30  ->  255.255.255.252   ->  11111111.11111111.11111111.11111100

Where the mask is 1, that’s the network. Where it’s 0, that’s the host.

Network Address vs Host Address

For 192.168.1.50/24:

  • Network address: 192.168.1.0 (host bits all zero)
  • Broadcast address: 192.168.1.255 (host bits all one)
  • Usable hosts: 192.168.1.1 – 192.168.1.254 (254 total)

The two reserved addresses (network + broadcast) cost us 2 IPs per subnet.

A Simple Subnetting Example

Say we get the block 10.0.0.0/24 (256 addresses) and want 4 equal subnets.

To get 4 subnets, we borrow log2(4) = 2 bits from the host portion. New prefix: /24 + 2 = /26.

Each /26 subnet has 2^(32-26) = 64 addresses (62 usable).

10.0.0.0/26       ->  10.0.0.0   – 10.0.0.63    (broadcast .63)
10.0.0.64/26      ->  10.0.0.64  – 10.0.0.127   (broadcast .127)
10.0.0.128/26     ->  10.0.0.128 – 10.0.0.191   (broadcast .191)
10.0.0.192/26     ->  10.0.0.192 – 10.0.0.255   (broadcast .255)

Done. Four neat subnets, 62 hosts each.

Quick Math Cheat Sheet

Prefix   Hosts (usable)   Common use
/30      2                Point-to-point links
/29      6                Tiny subnets
/28      14               Small office segment
/27      30               Floor of a building
/24      254              Typical LAN
/22      1022             Larger office
/16      65,534           Big private network
/8       16,777,214       Massive (10.0.0.0/8)

Formula: usable hosts = 2^(32 - prefix) - 2.

Why Subnet at All?

  • Security — keep finance servers off the same broadcast domain as the guest Wi-Fi.
  • Performance — smaller broadcast domains = less ARP/DHCP noise.
  • Routing efficiency — routers can summarize routes (longest prefix match).
  • IP conservation — give a /30 to a router-to-router link instead of a wasteful /24.

CIDR Aggregates Routes Too

CIDR isn’t just about splitting — it lets us combine adjacent networks:

192.168.0.0/24
192.168.1.0/24    ->  can be summarized as 192.168.0.0/23

Routing tables stay smaller. The internet’s BGP relies on this heavily.

Practical Tools

# Linux: ipcalc (install via apt/brew)
ipcalc 192.168.1.0/26
# Network:   192.168.1.0/26
# HostMin:   192.168.1.1
# HostMax:   192.168.1.62
# Broadcast: 192.168.1.63
# Hosts/Net: 62

# Or use Python
python3 -c "import ipaddress; n = ipaddress.ip_network('192.168.1.0/26'); print(list(n.hosts())[:5])"

Common Gotcha

The mask /31 is special — RFC 3021 allows /31 on point-to-point links with 2 usable hosts (no network/broadcast). Without that exception, /31 would have 0 usable hosts. /30 is still the safer textbook answer.

Interview Tip

Practice writing out the binary mask quickly. /24 = 255.255.255.0 should be reflex. For trickier prefixes like /27, remember the host bits: 5 bits of host = 32 hosts = mask 255.255.255.224 (256 - 32 = 224).

References