IsPrivate extension method for IPAddress

IPAddressExtensions.cs

using System;
using System.Net;
using System.Net.Sockets;
    
namespace MyNamespace
{
    /// <summary>
    /// Extension methods on <see cref="System.Net.IPAddress"/>.
    /// </summary>
    public static class IPAddressExtensions
    {
        /// <summary>
        /// Returns true if the IP address is in a private range.<br/>
        /// IPv4: Loopback, link local ("169.254.x.x"), class A ("10.x.x.x"), class B ("172.16.x.x" to "172.31.x.x") and class C ("192.168.x.x").<br/>
        /// IPv6: Loopback, link local, site local, unique local and private IPv4 mapped to IPv6.<br/>
        /// </summary>
        /// <param name="ip">The IP address.</param>
        /// <returns>True if the IP address was in a private range.</returns>
        /// <example><code>bool isPrivate = IPAddress.Parse("127.0.0.1").IsPrivate();</code></example>
        public static bool IsPrivate(this IPAddress ip)
        {
            // Map back to IPv4 if mapped to IPv6, for example "::ffff:1.2.3.4" to "1.2.3.4".
            if (ip.IsIPv4MappedToIPv6)
                ip = ip.MapToIPv4();

            // Checks loopback ranges for both IPv4 and IPv6.
            if (IPAddress.IsLoopback(ip)) return true;

            // IPv4
            if (ip.AddressFamily == AddressFamily.InterNetwork)
                return IsPrivateIPv4(ip.GetAddressBytes());

            // IPv6
            if (ip.AddressFamily == AddressFamily.InterNetworkV6)
            {
                return ip.IsIPv6LinkLocal ||
#if NET6_0_OR_GREATER
                        ip.IsIPv6UniqueLocal ||
#endif
                        ip.IsIPv6SiteLocal;
            }

            throw new NotSupportedException(
                    $"IP address family {ip.AddressFamily} is not supported, expected only IPv4 (InterNetwork) or IPv6 (InterNetworkV6).");
        }

        private static bool IsPrivateIPv4(byte[] ipv4Bytes)
        {
            // Link local (no IP assigned by DHCP): 169.254.0.0 to 169.254.255.255 (169.254.0.0/16)
            bool IsLinkLocal() => ipv4Bytes[0] == 169 && ipv4Bytes[1] == 254;

            // Class A private range: 10.0.0.0 – 10.255.255.255 (10.0.0.0/8)
            bool IsClassA() => ipv4Bytes[0] == 10;

            // Class B private range: 172.16.0.0 – 172.31.255.255 (172.16.0.0/12)
            bool IsClassB() => ipv4Bytes[0] == 172 && ipv4Bytes[1] >= 16 && ipv4Bytes[1] <= 31;

            // Class C private range: 192.168.0.0 – 192.168.255.255 (192.168.0.0/16)
            bool IsClassC() => ipv4Bytes[0] == 192 && ipv4Bytes[1] == 168;

            return IsLinkLocal() || IsClassA() || IsClassC() || IsClassB();
        }
    }
}

IPAddressExtensionsTests.cs

using System.Diagnostics.CodeAnalysis;
using System.Net;
using MyNamespace;
using FluentAssertions;
using Xunit;

namespace MyNamespace.Tests
{
    [SuppressMessage("ReSharper", "InvokeAsExtensionMethod")]
    public class IPAddressExtensionsTests
    {
        [Theory]
        [InlineData("1.1.1.1"     )] // Cloudflare DNS
        [InlineData("8.8.8.8"     )] // Google DNS
        [InlineData("20.112.52.29")] // microsoft.com
        public void IsPrivate_ReturnsFalse_PublicIPv4(string ip)
        {
            var ipAddress = IPAddress.Parse(ip);
            IPAddressExtensions.IsPrivate(ipAddress).Should().BeFalse();
        }

        [Theory]
        [InlineData("::ffff:1.1.1.1"     )] // Cloudflare DNS
        [InlineData("::ffff:8.8.8.8"     )] // Google DNS
        [InlineData("::ffff:20.112.52.29")] // microsoft.com
        public void IsPrivate_ReturnsFalse_PublicIPv4MappedToIPv6(string ip)
        {
            var ipAddress = IPAddress.Parse(ip);
            IPAddressExtensions.IsPrivate(ipAddress).Should().BeFalse();
        }

        [Theory]
        [InlineData("127.0.0.1"      )] // Loopback IPv4 127.0.0.1 - 127.255.255.255 (127.0.0.0/8)
        [InlineData("127.10.20.30"   )]
        [InlineData("127.255.255.255")]
        [InlineData("10.0.0.0"       )] // Class A private IP 10.0.0.0 – 10.255.255.255 (10.0.0.0/8)
        [InlineData("10.20.30.40"    )]
        [InlineData("10.255.255.255" )]
        [InlineData("172.16.0.0"     )] // Class B private IP 172.16.0.0 – 172.31.255.255 (172.16.0.0/12)
        [InlineData("172.20.30.40"   )]
        [InlineData("172.31.255.255" )]
        [InlineData("192.168.0.0"    )] // Class C private IP 192.168.0.0 – 192.168.255.255 (192.168.0.0/16)
        [InlineData("192.168.30.40"  )]
        [InlineData("192.168.255.255")]
        [InlineData("169.254.0.0"    )] // Link local (169.254.x.x)
        [InlineData("169.254.30.40"  )]
        [InlineData("169.254.255.255")]
        public void IsPrivate_ReturnsTrue_PrivateIPv4(string ip)
        {
            var ipAddress = IPAddress.Parse(ip);
            IPAddressExtensions.IsPrivate(ipAddress).Should().BeTrue();
        }

        [Theory]
        [InlineData("::ffff:127.0.0.1"      )] // Loopback IPv4 127.0.0.1 - 127.255.255.254 (127.0.0.0/8)
        [InlineData("::ffff:127.10.20.30"   )]
        [InlineData("::ffff:127.255.255.254")]
        [InlineData("::ffff:10.0.0.0"       )] // Class A private IP 10.0.0.0 – 10.255.255.255 (10.0.0.0/8)
        [InlineData("::ffff:10.20.30.40"    )]
        [InlineData("::ffff:10.255.255.255" )]
        [InlineData("::ffff:172.16.0.0"     )] // Class B private IP 172.16.0.0 – 172.31.255.255 (172.16.0.0/12)
        [InlineData("::ffff:172.20.30.40"   )]
        [InlineData("::ffff:172.31.255.255" )]
        [InlineData("::ffff:192.168.0.0"    )] // Class C private IP 192.168.0.0 – 192.168.255.255 (192.168.0.0/16)
        [InlineData("::ffff:192.168.30.40"  )]
        [InlineData("::ffff:192.168.255.255")]
        [InlineData("::ffff:169.254.0.0"    )] // Link local (169.254.x.x)
        [InlineData("::ffff:169.254.30.40"  )]
        [InlineData("::ffff:169.254.255.255")]
        public void IsPrivate_ReturnsTrue_PrivateIPv4MappedToIPv6(string ip)
        {
            var ipAddress = IPAddress.Parse(ip);
            IPAddressExtensions.IsPrivate(ipAddress).Should().BeTrue();
        }

        [Theory]
        [InlineData("::1"              )]  // Loopback
        [InlineData("fe80::"           )]  // Link local
        [InlineData("fe80:1234:5678::1")]  // Link local
        [InlineData("fc00::"           )]  // Unique local, globally assigned.
        [InlineData("fc00:1234:5678::1")]  // Unique local, globally assigned.
        [InlineData("fd00::"           )]  // Unique local, locally assigned.
        [InlineData("fd12:3456:789a::1")]  // Unique local, locally assigned.
        public void IsPrivate_ReturnsTrue_PrivateIPv6(string ip)
        {
            var ipAddress = IPAddress.Parse(ip);
            IPAddressExtensions.IsPrivate(ipAddress).Should().BeTrue();
        }

        [Theory]
        [InlineData("2606:4700:4700::64"                     )] // Cloudflare DNS
        [InlineData("2001:4860:4860::8888"                   )] // Google DNS
        [InlineData("2001:0db8:85a3:0000:0000:8a2e:0370:7334")] // Commonly used example.
        public void IsPrivate_ReturnsFalse_PublicIPv6(string ip)
        {
            var ipAddress = IPAddress.Parse(ip);
            IPAddressExtensions.IsPrivate(ipAddress).Should().BeFalse();
        }
    }
}

Line 37 should check for NET6_0_OR_GREATER instead of NET6_0 to avoid regressions when targeting newer frameworks.

That of course would be caught by your tests under most circumstances but I still wanted to point out the fact/fix.

@tskarman Thank you, I updated the gist.

These inline data parameters do not match the assertion in method IsPrivate_ReturnsTrue_PrivateIPv6
[InlineData("fe80::" )] // Link local
[InlineData("fe80:1234:5678::1")] // Link local
[InlineData("fc00::" )] // Unique local, globally assigned.
[InlineData("fc00:1234:5678::1")] // Unique local, globally assigned.

@Zakhar32 I don't follow, all those inline data parameters are different kinds of private IPs are they not?

And the assertion says IsPrivate(ipAddress).Should().BeTrue(), which matches the method name IsPrivate_ReturnsTrue_PrivateIPv6.