Fitting a Hospital's Growth into a Single /22
Meridian Regional Hospital has been assigned one block, 192.168.40.0/22, for a new clinical wing and may not use anything outside it. Facilities requires a separate VLAN for each of the following:
- Clinical workstations: 380 hosts today, and the CIO insists on 25% headroom inside the same subnet.
- Imaging and engineering: 180 hosts.
- Warehouse and logistics: 90 hosts.
- Guest Wi-Fi: 45 hosts.
- Three routed WAN links to outlying clinics, each needing exactly two usable addresses.
A partner VPN already uses 192.168.44.0/24 behind the firewall, so nothing in the plan may overlap it. The network manager wants each VLAN summarised in one line on the core, and the auditor wants documented, minimally wasted allocations. The block is currently unused and no device has been configured yet.
1. Produce the complete address plan: subnet and prefix, usable host range, and broadcast address for every segment.
Read the worked answer
| Segment | Subnet | Usable range | Broadcast |
|---|---|---|---|
| Clinical workstations | 192.168.40.0/23 | 192.168.40.1 - 192.168.41.254 | 192.168.41.255 |
| Imaging/engineering | 192.168.42.0/24 | 192.168.42.1 - 192.168.42.254 | 192.168.42.255 |
| Warehouse | 192.168.43.0/25 | 192.168.43.1 - 192.168.43.126 | 192.168.43.127 |
| Guest Wi-Fi | 192.168.43.128/26 | 192.168.43.129 - 192.168.43.190 | 192.168.43.191 |
| WAN link 1 | 192.168.43.192/30 | 192.168.43.193 - 192.168.43.194 | 192.168.43.195 |
| WAN link 2 | 192.168.43.196/30 | 192.168.43.197 - 192.168.43.198 | 192.168.43.199 |
| WAN link 3 | 192.168.43.200/30 | 192.168.43.201 - 192.168.43.202 | 192.168.43.203 |
Allocation rule used: largest requirement first, each subnet placed on its natural boundary so the prefix is valid. 192.168.40.0/22 spans 192.168.40.0 to 192.168.43.255, so nothing touches 192.168.44.0/24. The plan summarises cleanly on the core as 192.168.40.0/22.
2. Justify why the clinical VLAN needs a /23 rather than a /24, and show whether the three /30 WAN links fit in what remains.
Read the worked answer
Headroom arithmetic: 380 hosts with 25% growth is 380 x 1.25 = 475 hosts. A /24 provides 254 usable addresses (256 minus network and broadcast), which fails immediately. A /23 provides 510 usable addresses, so 475 fits with 35 addresses to spare - tight but correct, and it also gives room for the growth to be uneven across subnets later.
The /23 must start on an even third octet for alignment, so 192.168.40.0/23 consumes 192.168.40.0-192.168.41.255 and leaves 192.168.42.0 onward.
After the /24, /25 and /26 allocations, the free space is 192.168.43.192-192.168.43.255, a /26 = 64 addresses. Three /30s need 12 addresses (4 each, of which 2 are usable). They take 192.168.43.192, .196 and .200. Remaining spare: 192.168.43.204-192.168.43.255, that is 52 addresses (five further /30s plus a /27). So yes, the WAN links fit, and there is still room for future point-to-point links without leaving the block.
3. Six months later a 300-host research department is funded. Can it go inside the existing block without renumbering anything? Show the arithmetic and recommend what to do.
Read the worked answer
A 300-host subnet needs a /23 (510 usable); a /24 gives only 254. The /23 must be aligned on an even third octet, so candidates are 192.168.40.0/23 (occupied by clinical), 192.168.42.0/23 (which would span 192.168.42.0-192.168.43.255, occupied by imaging, warehouse, guest and the WAN links), and 192.168.44.0/23 (outside the block and overlapping the partner VPN).
The largest aligned free block today is a /27 (32 addresses). So the answer is no - a 300-host subnet cannot be added without renumbering production VLANs, which is exactly the outcome the CIO's headroom requirement was meant to avoid.
Recommendation: request a new block (a /23 or /22) and route it separately from the core, summarised as its own line. Do not renumber clinical or imaging to make room; renumbering is higher risk than adding a routed prefix. Also revisit the growth assumption - 25% headroom on one VLAN but none reserved for a new department was the planning gap.