Subnetting

One /24 gives you 254 hosts in a single broadcast domain. That's rarely what you want: sales, engineering and guests shouldn't share one flat network. Subnetting splits a network into smaller ones by moving the line between the network and host bits.

Borrowing host bits

Every bit you move from the host side to the network side doubles the number of subnets and halves the hosts in each one. Try it — drag the prefix, change the address, borrow bits:

Subnet calculator

IP addressPrefix length/24

192

11000000

168

10101000

10

00001010

0

00000000

Network — 24 bitsHost — 8 bits

Subnet mask

255.255.255.0

Network address

192.168.10.0

Broadcast

192.168.10.255

Usable hosts

254

Usable range

192.168.10.1 – 192.168.10.254

Borrow bits to create subnets

−2+

4 subnets · /26 · 62 usable hosts each

#	Network	Usable range	Broadcast
1	192.168.10.0/26	192.168.10.1 – 192.168.10.62	192.168.10.63
2	192.168.10.64/26	192.168.10.65 – 192.168.10.126	192.168.10.127
3	192.168.10.128/26	192.168.10.129 – 192.168.10.190	192.168.10.191
4	192.168.10.192/26	192.168.10.193 – 192.168.10.254	192.168.10.255

Two formulas cover nearly every exam question:

• Subnets = 2ⁿ, where n is the number of borrowed bits
• Usable hosts per subnet = 2ʰ − 2, where h is the number of host bits left

A worked example

Your company has four departments and one network: 192.168.10.0/24.

1. You need 4 subnets. 2² = 4, so borrow 2 bits.
2. The new prefix is 24 + 2 = /26, mask 255.255.255.192.
3. That leaves 6 host bits: 2⁶ − 2 = 62 hosts per department.
4. The subnets start every 64 addresses: .0, .64, .128, .192.

Quick reference

• /25 — mask 255.255.255.128, blocks of 128, 126 hosts
• /26 — mask 255.255.255.192, blocks of 64, 62 hosts
• /27 — mask 255.255.255.224, blocks of 32, 30 hosts
• /28 — mask 255.255.255.240, blocks of 16, 14 hosts
• /29 — mask 255.255.255.248, blocks of 8, 6 hosts
• /30 — mask 255.255.255.252, blocks of 4, 2 hosts (classic point-to-point link)

In the lab, you'll subnet a /24 for a three-site company and configure the router interfaces in Packet Tracer.