What Is DHCP, and How Do Phones Get an Address?

What DHCP Does

To work on a network, every device needs at least four details: its own IP address, a network mask (which says how large the local network is), the gateway address — the router through which it reaches the internet, and the address of a DNS server.

Once, all of these were typed in by hand on every computer. DHCP (Dynamic Host Configuration Protocol) does it automatically: the device connects, shouts to the network "I'm new, who gives me an address?", and the DHCP server — usually the router — answers with all the details.

You can compare it to a hotel receptionist: a guest arrives, the receptionist checks which room is free, gives them a key and explains where the dining room is. The guest doesn't need to know in advance which room is theirs.

The Four Steps

The conversation between the device and the server is very short and made of four steps, easy to remember by the word DORA:

All of this happens in under a second, every time you connect a phone or computer to the network or turn it on.

Renting, Not Selling: The Lease Time

DHCP doesn't give an address forever — it "rents" it for a set time, called the Lease. In home and office routers this is usually a day, sometimes a few hours and sometimes a week.

In the middle of the period the device asks to renew, and the server almost always agrees and leaves it the same address. Only if the device is off for a long time and the lease expires, the server may give the address to another device, and the first device will get a new address when it comes back.

For most devices this makes no difference. But some devices should always stay on the same address — a printer that everyone sends to, a camera, or a phone that is managed remotely. For them there is address reservation.

Address Reservation

In almost every router you can set it so that a specific device always gets the same address. The setting is called DHCP Reservation, "Static Lease" or "Reserved Address", and it is done by the device's MAC address — the permanent identifier burned into it.

The advantage over typing a fixed address into the phone itself: the setting lives in one place, in the router. If you replace the router or change the address range, you don't need to touch every phone. And the phone keeps getting its other details, such as DNS, from it automatically.

Entering an address manually in the device (a static address) also works, but it is error-prone: if the address you typed is within the range the DHCP hands out, one day another device will get the same address, and both will start dropping on and off.

How to do it in practice: log in to the router's admin screen, find the list of connected devices, identify the phone by its MAC address (it is printed on a sticker on the back of the phone), and choose "Reserve address" or "Add to the fixed list". The next time the phone asks for an address, it will get the same one. It is recommended to write the reserved addresses in a simple table next to the communications closet, together with the workstation name.

The Address Range — How Many Devices Fit

The DHCP server hands out addresses from a defined range (Pool), for example from 192.168.1.100 to 192.168.1.200. That is 101 addresses. If the range fills up, a new device simply won't get an address and won't connect.

This happens more than people think. An office of 15 employees easily has 60–80 devices today: computers, desk phones, employees' and guests' mobiles, printers, cameras, a TV in the meeting room and smart air conditioners. In a yeshiva or institution with guest Wi-Fi the number can be in the hundreds.

When the network is large, it is common to separate the phones onto their own network (VLAN), with its own address range. That way guests' devices don't "steal" addresses from the phones.

DHCP Options: More Than an Address

Beyond the address, DHCP can give the device more details, called options, which are numbered. Most are invisible, but a few are especially important for phones:

Option 66 is the tool that enables large-scale automatic provisioning: you connect a new phone, it asks the DHCP where to download settings from, downloads a file that matches its MAC address — and starts working as an extension, without anyone touching its keypad.

Two DHCP Servers on the Same Network — The Trap

A network should have only one DHCP server. When there are two, each device gets an address from whichever answers first, and sometimes from the wrong server — with a different router, different DNS, and sometimes on a completely different network.

This usually happens by accident: someone plugs a home Wi-Fi router into a jack in the office to "boost the internet" in a remote room, and that router starts handing out its own addresses. The result looks strange: some phones work and some don't, and sometimes the same phone works in the morning and stops in the afternoon.

The solution: set the extra router as an "Access Point" only, with DHCP turned off, or replace it with a dedicated access point. Managed switches can also block unauthorized DHCP servers (DHCP Snooping).

A Field Example: The Phone That Got a Strange Address

In a small law office (fictitious name: "Levy & Co.") three phones in the meeting room showed "Not registered", while the rest of the office worked. On the info screen of one of them, the address 10.0.0.15 appeared, while the whole office works on 192.168.1.x addresses.

It turned out that an old guest Wi-Fi router had been installed in the meeting room, connected to the office network, and its DHCP was active. The three phones in the room got an address and a DNS server from it that didn't work. Turning off the DHCP on the extra router and restarting the phones solved it.

How to See What the Device Received

• On an IP phone — in the "Status" or "Info" menu, or by pressing a particular button (it varies between models). The address, gateway and DNS appear, and sometimes also whether the address came from DHCP or was set manually.
• On a Windows computer — in the command line, ipconfig /all. All the details appear, including the DHCP server's address and when the lease expires.
• On the router — the admin screen usually has a list of all devices that received an address, with their MAC addresses. This is the convenient place to identify a phone and reserve an address for it.

An address that starts with 169.254 means the device did not get an address from DHCP at all, and chose an "emergency" address for itself. That is a sign that it isn't reaching the DHCP server — a cable, a switch, or a full range.

DHCP and the Phone Connected Through the Computer

Most desk phones have two network ports: one for the wall and one for the computer. That way one cable is enough for a workstation — the phone passes the network on to the computer.

As far as DHCP is concerned, these are two separate devices: the phone gets its own address, and the computer behind it gets its own address. Both take up room in the range, so in an office with 20 such workstations you need at least 40 addresses just for them.

When the phones are on a separate network (VLAN), the phone knows to put itself on the phone network and the computer on the regular network. This setting is sometimes also delivered through DHCP or through the switch.

What Happens When DHCP Is Unavailable

If the router is off or stuck, new devices won't get an address. Devices that already got an address usually keep working until their lease expires — so a DHCP failure sometimes appears gradually, not all at once.

After a power outage, all the devices ask for an address at the same moment. If the phones come up before the router, they may get stuck on a 169.254 address until they try again. Most phones retry on their own; if not, a brief power disconnect solves it.

A UPS for the router and the switch keeps the network alive during short outages, so the phones don't lose their address and registration at all.

Checklist: DHCP in an Office with Telephony

1. Only one DHCP server on the network — usually the main router.
2. An address range large enough for all the devices, with room to grow.
3. A working DNS delivered through the DHCP.
4. Reserved addresses for devices that must not move: printers, cameras, switches and access points.
5. Manual addresses — only outside the range the DHCP hands out.
6. Additional routers in the office — as access points, with DHCP turned off.
7. In a large place — a separate network for guests and for phones.