The demarc, short for demarcation point, is where the carrier's network ends and yours begins. The carrier is responsible for everything up to it, and its SLA stops there; you and the building are responsible for everything after it. In a multi-tenant building the demarc often sits in the main telecom room on the ground floor or in the basement, which means someone has to run cable from there to your suite. That run is the demarc extension, and it is one of the most common things left out of an install plan.
Nobody thinks about the demarc until a circuit goes down and the carrier says its side tests fine. At that point, knowing exactly where the line sits, and who owns each piece of cable on your side of it, decides how long the outage lasts.
What the demarc is, physically
On paper it is a boundary. In the building it is a specific port you can point to: the customer-facing port on the carrier's network interface device, a labelled position on a patch panel, or a jack on the wall. For older copper telephone lines it was often a box called a smart jack. For fiber services it is usually a small carrier device with a fiber coming in one side and an Ethernet or fiber port for you on the other.
The demarc is often in the building's main point of entry, sometimes called the MPOE or main telecom room, where carriers bring their cables into the building. In a single-tenant building it may be in your own comms room from day one.
Why the location matters
- Repair responsibility. The carrier fixes faults up to the demarc. A cut cable between the basement and your floor is not its fault, and usually not its job, unless the contract says otherwise.
- The SLA. Uptime, latency and repair commitments are measured to the demarc. Our guide to what a 99.99% SLA buys lists the demarc among the standard exclusions.
- Testing. When you open a ticket, the carrier tests to its device. If it can reach the device, it will tell you the circuit is up, even if nothing in your office works.
- Cost. Cabling beyond the demarc is a line item someone pays for, and it is often missing from the quote you compared.
Where the demarc usually sits
In the US, rules that date from the deregulation of telephone service set where a carrier's wiring responsibility ends, and in multi-unit buildings the demarcation point is commonly at or near the main point of entry, with the building owner having a say in where it goes. Data circuits generally follow the same practice by convention and contract. The practical result:
- Single-tenant buildings: the demarc is usually inside the building near where the cable enters, often in the tenant's own equipment room.
- Multi-tenant office buildings: the demarc is usually in the main telecom room, and each tenant's service has to be carried from there to its suite.
- Buildings with floor telecom closets: some carriers will extend their own equipment to a closet on your floor, which shortens the run you are responsible for.
The only way to know for a specific circuit is to ask the carrier where it will terminate, and to see the survey notes.
Demarc extensions
A demarc extension is the cable run from the carrier's handoff to wherever your own equipment lives. Who does it varies:
- Some carriers include it in the install, sometimes up to a set distance.
- Some offer it as a separate charge.
- Some leave it to the customer, who hires a cabling contractor.
- Some buildings require their own approved contractor for any work in the riser.
Settle this early, in writing, because an install visit with no extension in place cannot hand you a working circuit in your suite. Our guide to installation day covers how this shows up in the survey.
Copper or fiber for the extension
Standard copper Ethernet cabling has a maximum channel length of 100 metres. That covers most single floors, but a run from a basement telecom room to an upper floor can exceed it once you add the horizontal distance. Longer runs use fiber, which then needs the right type (single-mode or multimode) and matching optics at each end, or a switch partway along to restart the copper distance. The carrier or contractor should state the cable type, the route and the connectors on each end.
The pathway
Extensions usually travel up the building riser, through floor closets and across ceilings. Building rules on fire-stopping, conduit and labelling apply. A crowded riser or a closet the building keeps locked can add days, so involve the property manager before the work is scheduled.
Who owns which piece
| Segment | Typical owner | Who you call when it fails |
|---|---|---|
| Carrier fiber from the street to its device | Carrier | The carrier's repair line |
| Carrier device at the demarc | Carrier | The carrier's repair line |
| Demarc extension to your suite | Whoever installed and paid for it, per the contract or lease | The cabling contractor, the building or the carrier, depending on who owns it |
| Building riser and closets | Building owner | Property management |
| Your firewall, switches, Wi-Fi and office cabling | You | Your IT person or managed provider |
The row that causes the most arguments is the extension. Write down who owns it at install, not during an outage.
Inside wiring beyond the demarc
Everything on your side is inside wiring: the patch panels, horizontal cabling to desks, access points and the switches that join them. It is yours to maintain, upgrade and document. It also has a lifecycle the circuit does not. Leases may say whether cabling stays when you leave, whether you must remove it, and who can use the riser. Old cable left behind by previous tenants is common, and it is why labelling matters so much. For larger buildings, our article on internet for multi-tenant buildings looks at risers and telecom rooms from the landlord's side.
A worked example
Hypothetical building and distances, for illustration.
A firm leases a suite on the 14th floor. The carrier is lit in the building, with its equipment in the basement telecom room, and the survey note reads "demarc in MPOE, customer to extend." The route up the riser is about 70 metres, and from the 14th-floor closet to the firm's comms cabinet is another 40 metres. At around 110 metres in total, a single copper Ethernet run is out of specification.
The firm has three options. Run fiber the full distance and fit matching optics at both ends. Ask the carrier whether it can place its device in the 14th-floor closet instead, so only the short horizontal run is the firm's. Or put a small switch in the floor closet to split the distance into two compliant copper runs, which adds another powered device to look after. They choose fiber, schedule the contractor through the building a week before the install, and label both ends with the circuit ID. On install day, the technician patches the circuit to the new fiber and the firm has service in its suite on the first visit.
How to document the demarc
- Record the demarc location: building, room, rack or wall position, and the port.
- Label the carrier device and both ends of the extension with the circuit ID.
- Photograph the device, the patch panel and the cable route where it is visible.
- Write down who installed the extension, the cable type and the date.
- Keep the carrier's repair number and the building's after-hours contact together with this record.
- Update it when anything moves, and hand it over when the office manager or IT provider changes.
Common demarc problems
- The extension was never ordered. Each party assumed another one was doing it.
- An unlabelled cable gets reused. A contractor working for another tenant takes what looks like a spare.
- Power in the telecom room. The carrier's device loses power because a building circuit is turned off, and nobody on your side can reach it.
- Mismatched optics. A fiber handoff meets a firewall with the wrong module, and the link never comes up.
- Nobody can get in. The telecom room is locked after hours, so a simple reset waits until morning.
A managed network provider can own this documentation for you and handle the call to the carrier or building when something on either side of the line fails.
Questions to ask your provider
- Where exactly will the demarc be for this circuit: which room, and which port?
- Is the demarc extension to my suite included, charged separately, or my responsibility?
- Can your equipment be placed on my floor instead of the main telecom room?
- What is the handoff: copper or fiber, and which connector and optic?
- If the extension fails, will you troubleshoot it, or only test to your device?
- Who in the building controls access to the telecom room out of hours?
Get the whole path quoted
A quote that stops at the demarc is only part of the price. When carriers bid on your address, we ask each one where it will terminate and what it will do about the extension, so the comparison includes the cable to your suite. Several Tier 1 carriers bid, the carrier pays FiberX, so there is no markup, and the quote is free with no obligation. Read about dedicated internet access, send us the address and floor, call 478-758-8091 or text (347) 870-0965.
