General

An IP address is composed of two parts: the network prefix in the high-order bits and the remaining bits called the host identifier.

  • IPv4 gives you a 32-bit address space → ~4.3 billion addresses
  • IPv6 gives you a 128-bit address space → ~340 undecillion addresses

Classful Networks (A, B, C)

Before CIDR existed, IP addresses were divided into fixed classes. The class was determined by the first few bits of the address.

Class First bits Range Default mask Networks Hosts per network Usage
A 0... 0.0.0.0 – 127.255.255.255 /8 (255.0.0.0) 128 16,777,214 Very large organizations
B 10... 128.0.0.0 – 191.255.255.255 /16 (255.255.0.0) 16,384 65,534 Large organizations
C 110... 192.0.0.0 – 223.255.255.255 /24 (255.255.255.0) 2,097,152 254 Small organizations
D 1110... 224.0.0.0 – 239.255.255.255 — — — Multicast (no host assignment)
E 1111... 240.0.0.0 – 255.255.255.255 — — — Reserved / experimental

Formula: Hosts per network = 2^(host bits) − 2 The − 2 accounts for the network address (all host bits = 0) and the broadcast address (all host bits = 1), which can never be assigned to a host.

Example — Class C (/24)

IP:   201.1.1.0
Mask: 255.255.255.0  →  24 bits network, 8 bits host
Hosts: 2^8 − 2 = 256 − 2 = 254
Network address:   201.1.1.0    (reserved)
Broadcast address: 201.1.1.255  (reserved)
Usable hosts:      201.1.1.1 – 201.1.1.254

Are classful networks still used today?

No — classful networking has been obsolete since 1993.

The problems were:

  • Wasteful: a company needing 300 hosts had to get a Class B (65,534 addresses), wasting ~65,000 addresses.
  • Inflexible: blocks came in only three sizes — too small (254), too big (65,534), or way too big (16 million).
  • IPv4 exhaustion: the rigid structure accelerated address depletion.

CIDR replaced it entirely. However, the private address ranges defined in that era (RFC 1918) are still universally used:

Range Classful origin Common use today
10.0.0.0/8 Class A Large private networks, cloud VPCs
172.16.0.0/12 Class B (partial) Enterprise networks
192.168.0.0/16 Class C Home routers, small offices

CIDR (Classless Inter-Domain Routing)

CIDR (RFC 1519, 1993) replaces fixed classes with a flexible /prefix notation. The prefix length tells you exactly how many bits are reserved for the network — the rest are available for hosts.

192.168.1.0/24
            └── 24 bits = network prefix
                8 bits  = host portion

Step-by-step: how to calculate usable IPs

Given: 192.168.1.0/24

Step 1 — How many bits for hosts?

Total bits:    32
Network bits:  24
Host bits:     32 − 24 = 8

Step 2 — Total addresses in the block

2^8 = 256 addresses

Step 3 — Subtract reserved addresses

256 − 2 = 254 usable host addresses

Step 4 — Identify the special addresses

Network address:   192.168.1.0    (first — all host bits = 0, identifies the subnet)
Broadcast address: 192.168.1.255  (last  — all host bits = 1, sends to all hosts)
Usable range:      192.168.1.1 – 192.168.1.254

Given: 198.51.100.0/22

Step 1 — Host bits

32 − 22 = 10 host bits

Step 2 — Total addresses

2^10 = 1024 addresses

Step 3 — Usable hosts

1024 − 2 = 1022 hosts

Step 4 — Address range

Network address:   198.51.100.0    (reserved)
Broadcast address: 198.51.103.255  (reserved)
Usable range:      198.51.100.1 – 198.51.103.254

Tip: to find the broadcast address, set all host bits to 1. /22 means the last 10 bits are host bits. 198.51.100.0 in binary ends in ...00 00000000 Set all host bits to 1 → ...11 11111111 = 198.51.103.255 ✓

Quick reference table

CIDR Host bits Total addresses Usable hosts Subnet mask
/8 24 16,777,216 16,777,214 255.0.0.0
/16 16 65,536 65,534 255.255.0.0
/24 8 256 254 255.255.255.0
/25 7 128 126 255.255.255.128
/26 6 64 62 255.255.255.192
/27 5 32 30 255.255.255.224
/28 4 16 14 255.255.255.240
/29 3 8 6 255.255.255.248
/30 2 4 2 255.255.255.252
/31 1 2 2* 255.255.255.254
/32 0 1 1* 255.255.255.255

* /31 is used for point-to-point links (RFC 3021) — no network/broadcast reservation needed. * /32 identifies a single host (e.g. a loopback or a static route to one IP).

Why CIDR matters

  • Efficient allocation: give a company exactly the block size they need (e.g. /26 for 62 hosts instead of a full /24).
  • Route aggregation (supernetting): multiple smaller blocks can be summarized into one route entry, keeping routing tables manageable.
  • Foundation of the modern internet: every cloud provider (AWS VPCs, Azure VNets, GCP subnets) uses CIDR notation exclusively.

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