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MaxMind DB Reader

NuGet API Docs


This is the .NET API for reading MaxMind DB files. MaxMind DB is a binary file format that stores data indexed by IP address subnets (IPv4 or IPv6).



We recommend installing this library with NuGet. To do this, type the following into the Visual Studio Package Manager Console:

install-package MaxMind.Db


Note: For accessing MaxMind GeoIP2 databases, we generally recommend using the GeoIP2 .NET API rather than using this package directly.

To use the API, you must first create a Reader object. The constructor for the reader object takes a string with the path to the MaxMind DB file. Optionally you may pass a second parameter with a FileAccessMode enum with the value MemoryMapped or Memory. The default mode is MemoryMapped, which maps the file to virtual memory. This often provides performance comparable to loading the file into real memory with the Memory mode while using significantly less memory.

To look up an IP address, pass a System.Net.IPAddress object to the Find<T> method on Reader. This method will return the result as type T. T may either be a generic collection or a class using the [MaxMind.Db.Constructor] attribute to declare which constructor to use during deserialization and the [MaxMind.Db.Parameter("name")] to map the database key name to a particular constructor parameter.

We recommend reusing the Reader object rather than creating a new one for each lookup. The creation of this object is relatively expensive as it must read in metadata for the file.

Example Decoding to a Dictionary

using (var reader = new Reader("GeoIP2-City.mmdb"))
    var ip = IPAddress.Parse("");
    var data = reader.Find<Dictionary<string, object>>(ip);

Example Decoding to a Model Class

using MaxMind.Db;
using System.Net;

namespace MyCode
    public class Asn
        public AsnResponse(
            // The Parameter attribute tells the reader to map the database
            // key to the specified constructor parameter.
            [Parameter("autonomous_system_number")] long? autonomousSystemNumber,
            [Parameter("autonomous_system_organization")] string autonomousSystemOrganization,

            // The Inject attribute allows you to inject arbitrary values
            // when deserializing.
            [Inject("ip_address")] IPAddress ipAddress),

            // The Network attribute tells the reader to set the constructor
            // parameter to be the network associated with the record in the
            // database.
            [Network] Network network


    public class Program
        private static void Main(string[] args)
            using (var reader = new Reader("GeoLite2-ASN.mmdb"))
                var ip = IPAddress.Parse("");
                var injectables = new InjectableValues();
                injectables.AddValue("ip_address", ip);
                var data = reader.Find<Asn>(ip, injectables);

Multi-Threaded Use

This API fully supports use in multi-threaded applications. In such applications, we suggest creating one Reader object and sharing that among threads.


The MaxMind DB format is an open format for quickly mapping IP addresses to records. See the specification for more information on the format.

Bug Tracker

Please report all issues with this code using the GitHub issue tracker.

If you are having an issue with a MaxMind database or service that is not specific to this reader, please contact MaxMind support.


Patches and pull requests are encouraged. Please include unit tests whenever possible.


The MaxMind DB Reader API uses Semantic Versioning.

Copyright and License

This software is Copyright (c) 2013-2022 by MaxMind, Inc.

This is free software, licensed under the Apache License, Version 2.0.