The fields shown on the area code, rate centre and LATA pages, in plain language.
- Rate centre — The exchange area a number belongs to
- The telephone exchange area a block of numbers is homed to, and what decides whether a call between two points is local. It is not a city: many places are localities inside a larger rate centre and have no numbers of their own. Clinton, Maryland is part of the Washington Zone 16 rate centre, so its numbers are 227, 240 and 301.
- NPA-NXX — Area code and prefix
- The first six digits of a North American number. NPA is the three-digit area code, NXX the three-digit prefix — so 416-200 is NPA 416, NXX 200. Knowing the NPA-NXX identifies the exchange a number was issued from.
- Thousands-block — A prefix split into ten pieces
- A prefix holds 10,000 numbers, which is more than most carriers need, so prefixes are pooled: split into ten blocks of a thousand and handed to different carriers. The block digit is the fourth digit of the number, and
A means the whole prefix went to one carrier. This is why 512 has 783 prefixes but 6,011 rows — 512-555-1xxx and 512-555-2xxx can belong to different companies. - OCN — The carrier holding the block
- Operating Company Number — a four-character code identifying the carrier of record for a block. Two carriers can share a brand name and still have different OCNs, which is why the tables show the number alongside the name.
- ILEC — The incumbent operator for the area
- Incumbent Local Exchange Carrier — the established local operator for a rate centre, usually the former Bell company. It differs from the OCN whenever a block belongs to a competitor: a Toronto block can be held by Bell Cellular while the incumbent for the rate centre is Bell Ontario.
- LATA — Where a call stops being local
- Local Access and Transport Area — the region a call is routed within before it becomes long-haul. The boundaries date from the 1984 Bell System breakup and still shape how calls are routed and rated. A LATA is three digits, though some rate centres are published with a two-digit sub-code appended, shown here as 460.17. Browse every LATA.
- LIR — The Canadian equivalent, roughly
- Local Interconnection Region — how Canadian carriers divide territory for exchanging traffic. It only applies in Canada, so this field is blank on every US record by definition rather than by omission.
- Switch / CLLI — The equipment serving the block
- The telephone switch a block is routed through, named by its CLLI code — an eleven-character identifier where the first characters encode the place, as in TOROON01DS0 for Toronto. A switch typed POI is a point of interconnection, where carriers hand traffic to each other rather than serve subscribers directly.
- Overlay and split — The two ways to add an area code
- A split divides a region and gives part of it a new code, so existing numbers change. An overlay adds a code on top of the same territory, so nobody has to change a number — but everyone has to dial ten digits, since two numbers in the same place can now differ only by area code. 786 overlaid 305 in 1998; 941 split from 813 in 1995.