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Merged
lduchosal merged 2 commits into from
Jan 29, 2026
Merged

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

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f0663c9
fix: ipnetwork / first last usable / inverted issue
lduchosal Jan 29, 2026
24a129f
fix: ipnetwork / first last usable / inverted issue
lduchosal Jan 29, 2026
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10 changes: 7 additions & 3 deletions src/System.Net.IPNetwork/IPNetwork2Members.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
}
}
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137 changes: 137 additions & 0 deletions src/TestProject/IPNetworkTest/IPNetworkSlash31Tests.cs
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Original file line number Diff line number Diff line change
@@ -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");
}
}
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