DHCP reservation: a fixed IP via your router

Your server should not play musical chairs with IP addresses every time the router restarts.

The short answer

A DHCP reservation tells your DHCP server to always hand the same IP address to one specific device, identified by its MAC address. The device still asks for its settings automatically, so you manage the address in one place, on the router or DHCP server, not on every device. Our assessment: reserve addresses for servers, printers and anything that a port forwarding rule or a bookmark points at.

How it works

1. What DHCP does. DHCP (RFC 2131) provides a framework for passing configuration to hosts on a TCP/IP network, including the automatic allocation of reusable network addresses [1]. A device that joins asks for an address and gets one for a limited time, a lease. In a home network the addresses come from the private ranges of RFC 1918 [4].
2. Three ways to assign. RFC 2131 distinguishes automatic allocation, dynamic allocation with leases, and manual allocation, where the administrator picks the address and DHCP only delivers it [1]. A reservation is the manual kind, kept on the server.
3. The key is the MAC address. The server matches the device's hardware address to the entry you saved. Our assessment: if a device changes its network card or uses a different adapter, it counts as a new device, so check the MAC address you list.
4. Reservation versus a static address on the device. With a static address the setting lives on the device and the DHCP server knows nothing about it, so the server may hand the same address to someone else. A reservation keeps the server informed. AVM's guide for the FRITZ!Box says the address you enter must not be used by another device and must lie in the box's own network [2].
5. Inside or outside the pool. AVM says the address may lie inside or outside the DHCP range, which is 20 to 200 in the factory setting of the FRITZ!Box [2]. Our assessment: pick addresses outside the pool, so the server never offers them to others by mistake.
6. Many routers already do it. AVM states that by default the FRITZ!Box assigns each device the same address, storing the MAC address with the address given first, and that it only hands out a new address if the maximum number of addresses in the network has been reached [2]. The option "Assign permanent IPv4 address" makes the promise explicit [2]. Other routers and DHCP servers differ, and their defaults are unverified here.
7. Why it matters. Port sharing needs a stable target: AVM lists only devices that obtain their settings from the box automatically, and describes a permanent address as the step for port sharing [2][3]. The same applies to monitoring, backups and DNS entries that name an address (see Related pages: port forwarding).

A homelab example

The Bytearium homelab runs rack servers (HPE ProLiant DL360 Gen9 and DL380 Gen10) and a QNAP NAS, as of 14 to 18 Oct 2026. We have not recorded how addresses are assigned, so we say nothing about it. A general walkthrough on a FRITZ!Box: open the home network overview, edit the device, and enable "Assign permanent IPv4 address". If you want a specific address, type it in, choose one that no other device uses [2]. Keep a small table with device name, MAC address, reserved address and date. Then restart the device and confirm that it comes back with the same address.

Checklist

• Write down the MAC address of the device's wired or wireless adapter.
• Check that the device obtains its settings automatically, not with a static entry [2].
• Choose an address in your network that no other device uses [2].
• Prefer an address outside the DHCP range.
• Enter the reservation on the router or DHCP server, not only on the device.
• Restart the device and confirm that the address stays.
• Keep a table of reserved addresses with the date.
• Update port forwarding rules and DNS entries when an address changes.