A Master Street Address Guide, usually called an MSAG, is the authoritative database of every valid street address within a 9-1-1 jurisdiction. It pairs each range of house numbers on each named street with the specific police, fire, and EMS agencies responsible for that location. When you dial 9-1-1 from a landline or a registered VoIP phone, the system checks your address against this database before it decides which dispatcher answers and which responders get sent.
What an MSAG Record Actually Contains
An MSAG record is more structured than a mailing address. Under the NENA standard data format, each record carries roughly twenty fields, but a handful do the routing work.1National Emergency Number Association. Standard Data Formats for E9-1-1 Data Exchange and GIS Mapping
- The street name itself, stripped of directional words. “North Elm Street” stores “Elm” in the name field.
- A prefix or post directional (N, S, E, W, NE, and so on). This is why 100 N Main St and 100 S Main St can route to different responders.
- A standardized street suffix following U.S. Postal Service Publication 28 conventions: AVE, BLVD, DR.
- A low and high house number range. If a segment runs from 100 to 198 Elm Ave, the system knows 150 Elm Ave is valid and 250 Elm Ave is not.
- An odd/even indicator telling the system whether the range covers one side of the street or both.
- A community name recognized by the local 9-1-1 authority, which may differ from the postal city. This is what keeps an “Oak Lane” call in one township from routing to a dispatcher in the neighboring township that also has an Oak Lane.
- An Emergency Service Number (ESN) linking the address range to specific police, fire, and EMS agencies.
- A PSAP ID identifying which Public Safety Answering Point handles the call.
One thing the MSAG deliberately leaves out is sub-addressing. Apartment numbers, suite letters, floor designations, and unit numbers never appear in the street name field. The MSAG works at the street-segment level, not the individual-unit level, so a separate validation database translates between a full civic address with “Apt 4B” and the MSAG-valid street entry.2National Emergency Number Association. NENA Data Standards for the Provisioning and Maintenance of MSAG Files to VDBs and ERDBs
How It Validates Your Address Before You Ever Call
Validation happens when a phone line is activated or a VoIP account is set up, not when you dial. The service provider submits the customer’s address for comparison against the MSAG. If the street name exists, the house number falls within a known range, and the community name lines up, the address is linked to the appropriate emergency routing data.
If it doesn’t match, the system generates what’s called a kick-out. The record bounces back to the provider as an error, and the address cannot enter the emergency routing system until someone fixes the mismatch. Sometimes it’s a typo or a nonstandard abbreviation. Sometimes the street genuinely isn’t in the MSAG yet because the development is new and the local 9-1-1 coordinator hasn’t added it. Until the discrepancy is resolved, the address sits in limbo, unroutable for emergency purposes.
The practical consequence matters. If you move into a brand-new development and your street hasn’t been entered yet, your 9-1-1 call may not route correctly until the data is added. The process is designed to catch these gaps before an emergency, but it depends on providers submitting records and coordinators keeping up with new construction.
VoIP and Mobile Complications
A traditional landline has a fixed physical location that maps cleanly to an MSAG record. VoIP and mobile phones don’t. A VoIP phone can be carried anywhere, so the address on file may not reflect where the caller actually is.
FCC rules require interconnected VoIP providers to obtain a Registered Location from each customer before activating service and to give customers a way to update that location themselves. If the provider can detect that a call is coming from somewhere other than the registered address, it must either prompt the customer to update or do so automatically.3Federal Communications Commission. Dispatchable Location for 911 Calls from Fixed Telephony, Interconnected VoIP, TRS, and Mobile Text Service When automated dispatchable location isn’t technically feasible, the provider falls back on the registered address, which still has to match an MSAG record to route correctly.
Federal law requires IP-enabled voice service providers to deliver 9-1-1 and enhanced 9-1-1 service and directs the FCC to develop best practices, including validation procedures for location information in the relevant databases.4Office of the Law Revision Counsel. 47 USC 615a-1 – Duty to Provide 9-1-1 and Enhanced 9-1-1 Service In practice, VoIP providers validate registered locations against the MSAG the same way traditional carriers do.
How the MSAG Works With the ALI Database
The MSAG and the Automatic Location Identification (ALI) database are two halves of the same system. The MSAG is the master template: every valid street, every valid range. The ALI database holds the individual records tying a specific phone number to a specific name at a specific address. When a 9-1-1 call comes in, the system pulls the caller’s ALI record and displays the address on the dispatcher’s screen.
For that to work, the address in the ALI record has to match a valid MSAG entry. If it doesn’t, the dispatcher may see a blank screen, a partial address, or an error message instead of a usable location. NENA’s data exchange standards govern how providers and database management systems format and exchange ALI records so they align with the MSAG structure.5911.gov. NENA Standard Data Formats for 9-1-1 Data Exchange and GIS Mapping
Neither database is static. People move, streets get renamed, new developments go up. NENA recommends a minimum match rate of 98% between the MSAG and the associated GIS or ALI data before a system is considered operational, with the goal of continuous improvement toward 100%.6NENA: The 9-1-1 Association. NENA 71-501-v1 – Synchronizing GIS Databases with MSAG and ALI Falling below that threshold means a meaningful number of 9-1-1 calls could display the wrong location or route to the wrong dispatcher.
How the ESN Decides Who Responds
Every address range in the MSAG carries an Emergency Service Number, and that ESN maps to an Emergency Service Zone, a geographic area served by a defined combination of police, fire, and EMS agencies. A single county may have dozens of ESNs because different neighborhoods fall under different fire districts, municipal police departments, or volunteer ambulance services.
When a street is added to the MSAG or a jurisdiction boundary changes, the ESN assignment must be updated right away. If an annexed neighborhood keeps its old ESN after being absorbed into a new city, calls from that area could route to the previous city’s dispatcher, who no longer has authority to send help there.
Boundary situations get tricky. Where an address range or jurisdiction straddles two service areas, the ESN assignment follows what practitioners call “ground truth”: whoever actually responds first to that location, based on terrain, road access, and mutual aid agreements. A neighborhood bisected by a county line might be assigned entirely to one county’s PSAP if that county’s fire station is three minutes away and the other is fifteen.
Who Keeps the MSAG Current
Maintenance is a shared job, with each party handling a different piece.
- Local 9-1-1 coordinators own the content. They add new streets, update address ranges, remove records for abandoned roads, and assign ESNs, coordinating with planning departments to stay ahead of new construction.
- Addressing authorities, usually municipal or county offices, officially assign street names and house numbers. The coordinator depends on timely notification from them.
- Telecommunications carriers submit customer addresses for MSAG validation when service is activated and work with the coordinator to resolve any mismatches.
- Database Management System providers act as intermediaries, processing and validating ALI records submitted by carriers against the MSAG, catching errors, and returning rejected records for correction.7National Emergency Number Association. NENA Private Switch E-9-1-1 Database Standard
The weak link is usually timing. A subdivision can be occupied for weeks or months before its streets appear in the MSAG if the addressing authority is slow to notify the coordinator or the coordinator is slow to process the update. Regular audits comparing the MSAG against GIS data, ALI records, and real-world development are the main tool for catching gaps. Jurisdictions that let the MSAG fall behind expose themselves to serious liability if a misrouted call delays emergency response.
Where Next Generation 9-1-1 Changes the Picture
The legacy infrastructure described above was built for landline telephones. Next Generation 9-1-1 (NG9-1-1) replaces it with an IP-based system where routing decisions are made from GIS data rather than tabular MSAG records. Two GIS-driven functions take over the MSAG’s role: the Location Validation Function (LVF) checks whether a civic address is valid, and the Emergency Call Routing Function (ECRF) uses GIS polygon boundaries to route the call to the correct PSAP.8National Emergency Number Association. NENA i3 Standard for Next Generation 9-1-1 NENA’s baseline description explicitly frames the LVF as a replacement for MSAG-based validation.9National Emergency Number Association. Baseline NG9-1-1 Description
No national deadline exists for completing the switch, and deployment varies widely by jurisdiction. The MSAG will remain a working part of the 9-1-1 system for years in areas that haven’t fully transitioned, even as GIS-based routing and validation gradually take over.10NENA: The 9-1-1 Association. NENA NG9-1-1 Guide for 9-1-1 Authorities