fix/369-firstusable-lastusable-31-network (#370)

* fix: ipnetwork / first last usable / inverted issue

Author: lduchosal

src/System.Net.IPNetwork/IPNetwork2Members.cs

@@ -96,34 +96,38 @@ public IPAddress Broadcast
 
     /// <summary>
     /// Gets first usable IPAddress in Network.
+    /// Per RFC 3021, /31 networks have no reserved network address.
     /// </summary>
     public IPAddress FirstUsable
     {
         get
         {
             BigInteger first = this.InternalNetwork;
             if (this.family == AddressFamily.InterNetwork
-                && this.Usable > 1)
+                && this.cidr < 31)
             {
-                first+= 1;
+                first += 1;
             }
+
             return ToIPAddress(first, this.family);
         }
     }
 
     /// <summary>
     /// Gets last usable IPAddress in Network.
+    /// Per RFC 3021, /31 networks have no reserved broadcast address.
     /// </summary>
     public IPAddress LastUsable
     {
         get
         {
             BigInteger last = this.InternalBroadcast;
             if (this.family == AddressFamily.InterNetwork
-                && this.Usable > 1)
+                && this.cidr < 31)
             {
                 last -= 1;
             }
+
             return ToIPAddress(last, this.family);
         }
     }

src/TestProject/IPNetworkTest/IPNetworkSlash31Tests.cs

@@ -0,0 +1,137 @@
+// <copyright file="IPNetworkSlash31Tests.cs" company="IPNetwork">
+// Copyright (c) IPNetwork. All rights reserved.
+// </copyright>
+
+namespace TestProject.IPNetworkTest;
+
+/// <summary>
+/// Tests for /31 networks (point-to-point links) as per RFC 3021.
+/// See GitHub issue #369: https://github.com/lduchosal/ipnetwork/issues/369
+/// </summary>
+[TestClass]
+public class IPNetworkSlash31Tests
+{
+    /// <summary>
+    /// Tests that FirstUsable and LastUsable are correct for /31 networks.
+    /// According to RFC 3021, /31 networks have 2 usable addresses with no
+    /// network or broadcast addresses reserved.
+    ///
+    /// Bug: In version 2.6, FirstUsable and LastUsable are inverted for /31 networks.
+    /// Example: 167.92.212.82/31
+    ///   Current (Wrong): FirstUsable = 167.92.212.83, LastUsable = 167.92.212.82
+    ///   Expected: FirstUsable = 167.92.212.82, LastUsable = 167.92.212.83
+    /// </summary>
+    [TestMethod]
+    [TestCategory("Parse")]
+    [TestCategory("RFC3021")]
+    public void TestSlash31_FirstUsable_LastUsable_Issue369()
+    {
+        // Arrange - using the exact example from issue #369
+        string ipaddress = "167.92.212.82/31";
+
+        // Expected values per RFC 3021
+        string expectedNetwork = "167.92.212.82";
+        string expectedNetmask = "255.255.255.254";
+        string expectedBroadcast = "167.92.212.83";
+        string expectedFirst = "167.92.212.82";
+        string expectedLast = "167.92.212.83";
+        string expectedFirstUsable = "167.92.212.82";
+        string expectedLastUsable = "167.92.212.83";
+        byte expectedCidr = 31;
+        uint expectedUsable = 2;
+
+        // Act
+        var ipnetwork = IPNetwork2.Parse(ipaddress);
+
+        // Assert
+        Assert.AreEqual(expectedNetwork, ipnetwork.Network.ToString(), "Network");
+        Assert.AreEqual(expectedNetmask, ipnetwork.Netmask.ToString(), "Netmask");
+        Assert.AreEqual(expectedBroadcast, ipnetwork.Broadcast.ToString(), "Broadcast");
+        Assert.AreEqual(expectedFirst, ipnetwork.First.ToString(), "First");
+        Assert.AreEqual(expectedLast, ipnetwork.Last.ToString(), "Last");
+        Assert.AreEqual(expectedCidr, ipnetwork.Cidr, "Cidr");
+        Assert.AreEqual(expectedUsable, ipnetwork.Usable, "Usable");
+        Assert.AreEqual(expectedFirstUsable, ipnetwork.FirstUsable.ToString(), "FirstUsable");
+        Assert.AreEqual(expectedLastUsable, ipnetwork.LastUsable.ToString(), "LastUsable");
+
+        // Critical assertion: FirstUsable must be <= LastUsable
+        Assert.IsTrue(
+            ipnetwork.FirstUsable.ToString().CompareTo(ipnetwork.LastUsable.ToString()) <= 0 ||
+            ipnetwork.FirstUsable.Equals(ipnetwork.LastUsable),
+            "FirstUsable must be less than or equal to LastUsable");
+    }
+
+    /// <summary>
+    /// Tests another /31 network to ensure the fix works for any /31 subnet.
+    /// </summary>
+    [TestMethod]
+    [TestCategory("Parse")]
+    [TestCategory("RFC3021")]
+    public void TestSlash31_AnotherNetwork()
+    {
+        // Arrange
+        string ipaddress = "10.0.0.0/31";
+
+        string expectedFirstUsable = "10.0.0.0";
+        string expectedLastUsable = "10.0.0.1";
+        uint expectedUsable = 2;
+
+        // Act
+        var ipnetwork = IPNetwork2.Parse(ipaddress);
+
+        // Assert
+        Assert.AreEqual(expectedUsable, ipnetwork.Usable, "Usable");
+        Assert.AreEqual(expectedFirstUsable, ipnetwork.FirstUsable.ToString(), "FirstUsable");
+        Assert.AreEqual(expectedLastUsable, ipnetwork.LastUsable.ToString(), "LastUsable");
+    }
+
+    /// <summary>
+    /// Tests that /32 networks still work correctly (host route - single address).
+    /// </summary>
+    [TestMethod]
+    [TestCategory("Parse")]
+    public void TestSlash32_StillWorksCorrectly()
+    {
+        // Arrange
+        string ipaddress = "192.168.1.1/32";
+
+        string expectedFirstUsable = "192.168.1.1";
+        string expectedLastUsable = "192.168.1.1";
+        uint expectedUsable = 1;
+
+        // Act
+        var ipnetwork = IPNetwork2.Parse(ipaddress);
+
+        // Assert
+        Assert.AreEqual(expectedUsable, ipnetwork.Usable, "Usable");
+        Assert.AreEqual(expectedFirstUsable, ipnetwork.FirstUsable.ToString(), "FirstUsable");
+        Assert.AreEqual(expectedLastUsable, ipnetwork.LastUsable.ToString(), "LastUsable");
+    }
+
+    /// <summary>
+    /// Tests that /30 networks still work correctly (traditional smallest subnet with usable hosts).
+    /// </summary>
+    [TestMethod]
+    [TestCategory("Parse")]
+    public void TestSlash30_StillWorksCorrectly()
+    {
+        // Arrange
+        string ipaddress = "192.168.1.0/30";
+
+        string expectedNetwork = "192.168.1.0";
+        string expectedBroadcast = "192.168.1.3";
+        string expectedFirstUsable = "192.168.1.1";
+        string expectedLastUsable = "192.168.1.2";
+        uint expectedUsable = 2;
+
+        // Act
+        var ipnetwork = IPNetwork2.Parse(ipaddress);
+
+        // Assert
+        Assert.AreEqual(expectedNetwork, ipnetwork.Network.ToString(), "Network");
+        Assert.AreEqual(expectedBroadcast, ipnetwork.Broadcast.ToString(), "Broadcast");
+        Assert.AreEqual(expectedUsable, ipnetwork.Usable, "Usable");
+        Assert.AreEqual(expectedFirstUsable, ipnetwork.FirstUsable.ToString(), "FirstUsable");
+        Assert.AreEqual(expectedLastUsable, ipnetwork.LastUsable.ToString(), "LastUsable");
+    }
+}