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Course 2: IP Subnetting and VLSMLesson 1.7 (7 of 8 in this course)12 of 98 in the CCNA series

VLSM for a real office

One address block, seven networks of very different sizes. Plan it the way a network engineer would.

Intermediate · 9 min read · Before this: Network, broadcast and usable hosts

After this lesson, you can plan an office from one block with VLSM: size each subnet, place it on a valid boundary, and leave room to grow.

The brief

A new office has been given 10.20.0.0/23 (512 addresses, 10.20.0.0 to 10.20.1.255). It needs:

NetworkDevices
Staff (VLAN 10)200
IP phones (VLAN 20)100
Guest Wi-Fi (VLAN 30)50
Servers and printers (VLAN 40)20
Management (VLAN 99)10
WAN link to ISP2
WAN link to branch2

Equal /25s would give only four subnets, and 126 hosts is too small for staff anyway. This is what VLSM is for. Try to plan it on paper first.

The method

  1. Size each network. Smallest prefix where 2h − 2 covers the devices (and the gateway, and growth).
  2. Sort largest first. Big blocks need big boundaries; placing them first keeps every block aligned with no gaps.
  3. Allocate from the next free address. Each block starts where the last one ended. Check it starts on a multiple of its own size.
  4. Record and check. Write the network, range and broadcast of each, and confirm nothing overlaps.
Show the finished plan
NetworkNeedsSubnetUsable rangeBroadcastSpare
Staff (VLAN 10)20010.20.0.0/2410.20.0.1 – 10.20.0.25410.20.0.25554
IP phones (VLAN 20)10010.20.1.0/2510.20.1.1 – 10.20.1.12610.20.1.12726
Guest Wi-Fi (VLAN 30)5010.20.1.128/2610.20.1.129 – 10.20.1.19010.20.1.19112
Servers and printers (VLAN 40)2010.20.1.192/2710.20.1.193 – 10.20.1.22210.20.1.22310
Management (VLAN 99)1010.20.1.224/2810.20.1.225 – 10.20.1.23810.20.1.2394
WAN link to ISP210.20.1.240/3010.20.1.241 – 10.20.1.24210.20.1.2430
WAN link to branch210.20.1.244/3010.20.1.245 – 10.20.1.24610.20.1.2470
Spare–10.20.1.248/2910.20.1.249 – 10.20.1.25410.20.1.2556
10.20.0.010.20.1.010.20.1.255
The /23 drawn to scale. Staff take half; phones a quarter; everything else shares the last quarter. The small blocks at the right end are the /27, /28, /30s and the spare /29.

Why largest first

A block must start on a multiple of its own size: a /25 (128 addresses) can start at .0 or .128, not at .64. Place the big blocks first and each smaller block lands on a valid boundary automatically, right after the previous one.

Start with the small ones instead and each bigger block has to skip ahead to its next boundary, leaving gaps. Allocate management, then the two WAN links, from 10.20.0.0 and they end at .23; the servers' /27 can't start at .24, so it jumps to .32 and .24 to .31 sit empty. In this brief the gaps happen to fit the spare /29, but in a tighter block that is how plans run out of room. Largest first avoids the question.

Decisions a real plan makes

  • The gateway needs an address. Staff need 200 devices plus the gateway: 201, still well inside a /24's 254.
  • Growth. Staff have 53 spare addresses. If the office expects 300 staff, a /23 for staff alone would be needed, and so a bigger block from the start.
  • Keep the spare together. The leftover /29 sits at the end, in one piece, ready for another link or a small VLAN.
  • Point-to-point links. /30s are the classic choice and what CCNA questions usually expect. /31s (RFC 3021) would save two addresses per link where both routers support them.

💡 The VLSM calculator produces a plan like this from a list of requirements. Use it to check your own plan, after you have made one.

Check yourself

Predict · scenario 1

The office adds a CCTV VLAN with 25 cameras. Where can it go without changing the existing plan?

Predict · scenario 2

Why is the IP phones' subnet 10.20.1.0/25 and not 10.20.0.200/25?

Predict · scenario 3

Which pair of subnets overlaps?

Practise

Plan your own home or lab network the same way, then compare it with the VLSM calculator. For more speed on the arithmetic behind it, use the subnetting practice drill.

Learn more: Find the Mistake