What is an IP address?

So what is an IP address?

IP stands for Internet Protocol. It's the set of rules that determines how information passes from one device to another: every message, picture or word in a phone call is broken into small "packets," and each packet carries the sender's address and the recipient's address. These addresses are IP addresses.

The closest comparison is the mail. For a letter to arrive, you need a street, a house number and an apartment. On a network, the IP address is the full address of the device, and the routers along the way are the post offices that pass the packet on according to the address — until it reaches its destination.

It's important to understand that an IP address belongs to a device on a particular network, not to the device itself forever. A laptop that moves from the office to home gets a different address in each place. What does stay fixed on the device is a different address, the hardware address (MAC), which is burned into its network card.

What an address looks like: IPv4 and IPv6

The familiar address is IPv4: four numbers between 0 and 255 separated by dots, for example 192.168.1.21 or 31.154.27.128. In total this system has a little more than four billion addresses. When it was designed, in the early 1980s, that seemed endless. Today, when every person has a connected phone, computer, TV and watch, it's simply not enough.

The long-term solution is IPv6 — much longer addresses, written with digits and letters and colons, for example 2001:db8:85a3::8a2e:370:7334. It has an almost unimaginable number of addresses, enough to give one to every device on Earth, and many more. Internet providers in Israel are starting to support it, but most offices still work mainly with IPv4.

The short-term solution, the one that has kept the internet going for decades, is the separation between private and public addresses — more on that in the next section.

Private and public addresses

Inside the office, every device gets a private address, visible only on the internal network. Three fixed ranges were set aside for this, and you'll run into them again and again:

• 192.168.x.x — the most common in homes and small offices.
• 10.x.x.x — common in large networks and organizations.
• 172.16.x.x to 172.31.x.x — less familiar, but also private.

Toward the internet, the whole office goes out with one public address — the router's. The same private address, say 192.168.1.21, can exist in thousands of offices at the same moment with no conflict, because it never goes outside. The translation between the private addresses and the public one is done by NAT, inside the router.

A public address, by contrast, is unique in the entire world, and it's given by the internet provider. It can be fixed or changing — the difference is explained in the article on static IP and dynamic IP.

How a device gets an address

In most offices nobody types addresses by hand. When an IP phone connects to the network cable, it sends a request: "I'm new here, who gives me an address?" A service called DHCP, usually running on the router, answers and assigns a free address, together with a few other details: who the gateway out is (the router), and which server translates website names into addresses (DNS).

This assignment is a loan — for a limited time. After hours or days the phone renews it, and usually gets the same address again. For devices that must have an address that doesn't move, like a shared printer or a server, you set up a "reservation" in the router or a manual address.

This explanation matters because a well-known problem comes from it: the range of addresses the DHCP hands out runs out. In an office with 30 phones, 30 computers, printers and guests' mobiles, a range that's too small means the 60th device simply won't get an address, and its phone will show "No network."

Why IP addresses matter for telephony

A VoIP call is really a stream of voice packets sent dozens of times a second between the phone and the PBX. For the voice to arrive, the PBX needs to know which address to send it to. And here's the problem: the phone knows only its own private address, and the cloud PBX sees only the office's public address.

When the translation between them doesn't work properly, you get the most common problem in office networks: you hear only one side of the call. You hear the customer, he doesn't hear you — because your voice is being sent to the wrong address. Another problem: the phone shows "Not registered," because its registration messages (SIP) don't come back to it.

• Two routers in a chain (called Double NAT) — for example a provider's router with another router behind it. Each translation layer adds a chance for error.
• Address conflict — two devices with the same private address, usually because someone set an address by hand inside the DHCP range. The network behaves strangely, on and off.
• A public address that changed — if something is locked to the old address, it will stop working.

How to find your address

An IP phone's private address usually appears in its status menu, and on some models it's enough to press the OK key to see it. On a Windows computer, the ipconfig command in the command window will show it.

The office's public address is what the world sees. The simple way to find it is to search "what is my ip" in a browser from any computer in the office — all the devices will show the same address, which is exactly the principle of NAT.

A practical tip: when you contact support about a phone problem, it's worth writing down in advance the device's private address and the office's public address. It saves half the call.

When something isn't working: sign, cause, solution

Most network problems related to addresses repeat themselves. The following table collects the common cases, from the sign you see on the phone to the action that solves it:

A rule of thumb for diagnosis: if a problem appears on all devices — the problem is almost always in the router or the DHCP. If it appears on one device — start with the cable and the device's own settings.

Example from the field: a law firm with 12 stations

A law firm in Jerusalem moved to a cloud PBX. Twelve phones, twelve computers, two printers, a Wi-Fi access point and a camera at the entrance. For the first two weeks everything worked. Then, on the day of a big meeting with clients who connected to the Wi-Fi, three phones showed "No network" — specifically the ones that had been restarted that morning.

The investigation showed that the provider's router came with a small DHCP range: from 192.168.1.100 to 192.168.1.150, only 51 addresses. Twenty-eight permanent devices, plus twenty guests' mobiles — and the range ran out. Phones that already held an address kept working; those that asked for a new address were left without.

The fix took a quarter of an hour, in two steps: widening the range to 192.168.1.50 through 192.168.1.250 (about 200 addresses), and "reserving" addresses in the router for the two printers and the camera — so they always get the same address, with no manual setup on the device and no risk of conflict. Later, when the office upgraded to a business router, the guests were moved to a separate network, so they can't use up the office's range and can't see the phones at all.

The lesson: an address problem doesn't look like an address problem. It looks like "the phone broke," and it shows up on a busy day. So it's worth checking the size of the range on setup day, not on the day of the failure.

What to tell your internet provider

When you talk with the internet provider's support about a telephony problem, the right questions save time. These are the sentences to prepare in advance:

• "Is my public address a real address, or am I behind the provider's NAT (CGNAT)?" — the answer determines whether systems can be locked to your address at all.
• "Can your router be switched to bridge mode, so that my router is the only one translating addresses?"
• "Is SIP ALG active on your router, and can it be turned off?" — on some providers' routers only the provider can change this.
• "What DHCP range does your router hand out, and can it be widened?"
• "Does the connection support IPv6, and is it enabled?" — not required for phones, but good to know.

Tip: when the provider asks "What's your address?", they usually mean the public address — the one you see when you search "what is my ip" — and not the computer's 192.168 address.

Numbers worth knowing

• 254 — the number of devices that fit in an ordinary office network of the 192.168.1.x type (subnet mask 255.255.255.0, also written /24). The first address is reserved for the network itself and the last for broadcast to everyone. For an ordinary office that's a lot; for an institution with hundreds of devices, a wider network is chosen.
• 65,535 — the number of ports on each address. The address says "which device," and the port says "which program inside it." Telephony uses port 5060 for managing the call and a range of ports for the voice.
• Hours to days — the length of a DHCP address "loan." Many routers hand out for one day. Shortening to a few minutes burdens the network; lengthening to a week makes things hard for new devices when the range is narrow.
• 3 — the addresses every device must have in order to work: its own address, the gateway (router) address and a DNS address. If one is missing, there's an internal network but no internet, and therefore no PBX.