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DHCP and Reservations

The one-sentence version: DHCP loans addresses to devices that join your network; a reservation pins one address to one device so servers and printers stop moving. Use reservations for things you reach by address, static IPs on the device only when the device demands it.

Leases in plain terms

When a device joins your network — Wi-Fi connected, cable plugged in — it shouts \"I need an address!\" (that's the DHCP discover). Your router's DHCP server answers with a lease: an address, the network mask, the gateway (the router itself), and the DNS server, plus an expiry time. Before the lease expires, the device quietly asks to renew. Nobody notices.

Why leases instead of permanent addresses? Because devices come and go. Your guest's phone doesn't need a permanent seat at the table; it borrows one for the evening. Typical home lease times are 12–24 hours. A device that sleeps past its lease (looking at you, printer) can wake up to find its old address given to someone else — and every bookmark, app, and port-forward rule pointing at the old address breaks.

The refresher, in two sentences: this is the DHCP half of Job 2 in the router model. If you want the protocol mechanics, the fundamentals guide has them.

Reservations: the right default

A DHCP reservation tells the router: \"whenever this device asks, always give it this address.\" It ties the device's MAC address (its hardware identifier, burned into the network card) to one IP. The device still uses DHCP — it doesn't know anything special happened — but it always lands in the same seat.

Why this beats the alternatives:

Approach How it works Downside
Nothing (plain DHCP) Address may change after sleeps Bookmarks and rules break
Reservation (router UI) Router always assigns the same IP None, for most cases
Static IP on the device Device ignores DHCP, uses a fixed IP You must track assignments by hand; two devices can collide

Give reservations to: servers, printers, cameras, anything you reach by address or that other rules point at. Don't bother for phones and laptops — let them roam the pool.

Setting one up (typical router UI)

Router interfaces differ — this is the shape they all share:

  1. Find your router's DHCP or LAN settings page (often under \"Advanced → LAN\" or \"Network → DHCP Server\").
  2. Find the list of connected devices (or \"DHCP clients\") and locate the device — match by its current IP or its name.
  3. Add a DHCP reservation / static lease: pick the device's MAC address, choose an IP outside the DHCP pool but inside your network — for example, with a pool of 192.168.1.100–192.168.1.199, use something like 192.168.1.10.
  4. Save. Reconnect the device (or wait for its lease to renew) so it picks up the reserved address.

Check: ask the router for its DHCP client list after the device reconnects — the device's MAC should now show your chosen address, and it will survive reboots and long sleeps.

The same thing in dnsmasq

If your DHCP server is a Linux box or a Pi (Pi-hole-style setups run dnsmasq or its equivalent), the reservation lives in a config file instead of a web UI. This snippet was validated with the real dnsmasq (dnsmasq --test reported \"syntax check OK\"):

# Example dnsmasq config: DHCP pool, one reservation, one local DNS entry
interface=eth0
dhcp-range=192.168.1.100,192.168.1.199,24h
dhcp-host=AA:BB:CC:DD:EE:FF,nas,192.168.1.10,infinite
address=/home.arpa/192.168.1.1

Line by line:

  • dhcp-range — the pool: 100 addresses (192.168.1.100 through 192.168.1.199), 24-hour leases.
  • dhcp-host — the reservation: the device whose MAC is AA:BB:CC:DD:EE:FF always gets 192.168.1.10 (named nas here) with an effectively permanent lease.
  • address= — a local DNS entry, covered in Names on Your LAN.
  • interface=eth0 — which network card serves DHCP; match it to your box.

Find a device's MAC address (typical output — yours will differ):

# Linux
ip link show | grep -A1 "state UP" | grep link/ether

link/ether aa:bb:cc:dd:ee:ff brd ff:ff:ff:ff:ff:ff

On Windows: ipconfig /all and look for \"Physical Address.\" On a Mac: System Settings → Wi-Fi → Details → the hardware (MAC) address. Most router UIs also show it next to each connected device.

When a static IP on the device is right

Rarely, but it happens: some devices (old printers, some cameras, a server whose DHCP client you don't trust) let you type an address directly into their settings. If you do this:

  1. Pick the address from outside the DHCP pool (else the router may hand \"your\" address to a guest's phone and you get a collision — two devices, one address, both broken).
  2. Enter the mask (255.255.255.0 for a /24), the gateway (your router, e.g. 192.168.1.1), and DNS (the router, or your local DNS server).
  3. Write it down. Static IPs are invisible to the router's DHCP list, so they're the first thing forgotten during the next troubleshooting session.

If the device supports DHCP at all, a reservation is the same result with none of the bookkeeping. Reserve first; go static only when the device gives you no choice.

What DHCP can't do for you

  • DHCP doesn't give names — it hands out numbers. Names are the next two chapters.
  • DHCP doesn't cross subnets by itself. Devices on your guest network get addresses from that network's pool (see Guest Networks and VLANs).
  • A reservation doesn't make a device reachable from the internet — that's NAT and firewall territory (Port Forwarding).

Next: DNS at Home