Static vs DHCP
When to fix an IP, reservations, lease time.
Learning objectives
- Decide when a device needs a static IP
- Configure DHCP reservations vs dynamic pools
- Choose sensible lease times
DHCP — automatic addressing
The Dynamic Host Configuration Protocol lets devices request an IP, subnet mask, gateway, and DNS servers at boot. Workshop Co.’s router runs a DHCP server for office and guest VLANs.
Static vs DHCP reservation
| Method | How | Best for |
|---|---|---|
| Dynamic DHCP | Any free IP from pool | Phones, guest laptops |
| DHCP reservation | MAC address → fixed IP in router | Printers, cameras, POS |
| True static | IP configured on device | Servers, gear that ignores DHCP |
Worked example — Workshop Co. reservations
| Device | MAC (example) | Reserved IP |
|---|---|---|
| Receipt printer | aa:bb:cc:dd:ee:01 | 192.168.10.40 |
| NVR | aa:bb:cc:dd:ee:70 | 192.168.10.128 |
| POS tablet | aa:bb:cc:dd:ee:60 | 192.168.10.61 |
Staff laptops stay in the dynamic pool 192.168.10.100–.199 with 24-hour lease.
Short leases (1h) help crowded guest Wi‑Fi recycle addresses. Long leases (24h+) reduce DHCP chatter on stable office LANs. After subnet changes, shorten lease temporarily to force refresh.
Exclude reserved IPs from the dynamic pool. If the pool includes .40 and you reserve .40 for a printer, you risk duplicate assignments.
Try it yourself — design a pool
Subnet 192.168.10.0/26. Gateway .1. Ten reserved devices (.10–.19). Write the DHCP pool range for everything else.
Answer
/26 usable hosts: .1–.62 (network .0, broadcast .63). Gateway .1. Reservations .10–.19. Pool could be 192.168.10.20–192.168.10.59 — leaving .60–.61 spare or for future static gear.
Quick quiz
- What four parameters does DHCP typically provide?
- Why prefer reservation over static IP on the printer itself?
- What happens when two devices get the same IP?
Answers
- IP address, subnet mask, default gateway, DNS servers (plus optional options).
- Central management on router; easier migration if subnet changes.
- IP conflict — intermittent connectivity, ARP flapping; one device must change IP.