diff --git a/src/client/Microsoft.Identity.Client/Instance/Discovery/NetworkMetadataProvider.cs b/src/client/Microsoft.Identity.Client/Instance/Discovery/NetworkMetadataProvider.cs
index 22ddd37ec3..12b4ae23d0 100644
--- a/src/client/Microsoft.Identity.Client/Instance/Discovery/NetworkMetadataProvider.cs
+++ b/src/client/Microsoft.Identity.Client/Instance/Discovery/NetworkMetadataProvider.cs
@@ -118,9 +118,13 @@ private Uri ComputeHttpEndpoint(Uri authority, RequestContext requestContext)
                 authority.Host :
                 AadAuthority.DefaultTrustedHost;
 
-            string instanceDiscoveryEndpoint = UriBuilderExtensions.GetHttpsUriWithOptionalPort(
-                $"https://{discoveryHost}/common/discovery/instance",
-                authority.Port);
+            string instanceDiscoveryEndpoint = $"https://{discoveryHost}/common/discovery/instance";
+            if (string.Equals(discoveryHost, authority.Host, StringComparison.OrdinalIgnoreCase))
+            {
+                instanceDiscoveryEndpoint = UriBuilderExtensions.GetHttpsUriWithOptionalPort(
+                    instanceDiscoveryEndpoint,
+                    authority.Port);
+            }
 
             requestContext.Logger.InfoPii(
                 () => $"Fetching instance discovery from the network from host {discoveryHost}. Endpoint {instanceDiscoveryEndpoint}. ",
diff --git a/src/client/Microsoft.Identity.Lab.Api/Http/MockHttpManagerExtensions.cs b/src/client/Microsoft.Identity.Lab.Api/Http/MockHttpManagerExtensions.cs
index 98a4438320..a4d5123301 100644
--- a/src/client/Microsoft.Identity.Lab.Api/Http/MockHttpManagerExtensions.cs
+++ b/src/client/Microsoft.Identity.Lab.Api/Http/MockHttpManagerExtensions.cs
@@ -40,7 +40,11 @@ public static MockHttpMessageHandler AddInstanceDiscoveryMockHandler(
                                            ? authorityURI.Host
                                            : AadAuthority.DefaultTrustedHost;
 
-                discoveryEndpoint = UriBuilderExtensions.GetHttpsUriWithOptionalPort($"https://{discoveryHost}/common/discovery/instance", authorityURI.Port);
+                discoveryEndpoint = $"https://{discoveryHost}/common/discovery/instance";
+                if (string.Equals(discoveryHost, authorityURI.Host, StringComparison.OrdinalIgnoreCase))
+                {
+                    discoveryEndpoint = UriBuilderExtensions.GetHttpsUriWithOptionalPort(discoveryEndpoint, authorityURI.Port);
+                }
             }
             else
             {
diff --git a/tests/Microsoft.Identity.Test.Unit/CoreTests/InstanceTests/AadAuthorityTests.cs b/tests/Microsoft.Identity.Test.Unit/CoreTests/InstanceTests/AadAuthorityTests.cs
index 8b7186044d..b65c09ba16 100644
--- a/tests/Microsoft.Identity.Test.Unit/CoreTests/InstanceTests/AadAuthorityTests.cs
+++ b/tests/Microsoft.Identity.Test.Unit/CoreTests/InstanceTests/AadAuthorityTests.cs
@@ -297,12 +297,14 @@ public void TenantlessAuthorityChanges()
 
         [TestMethod]
         //Test for bug #1292 (https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/issues/1292)
-        public void AuthorityCustomPortTest()
+        public void AuthorityCustomPortIsPreservedAndNotUsedForGlobalInstanceDiscoveryTest()
         {
             const string customPortAuthority = "https://localhost:5215/common/";
 
             using var harness = CreateTestHarness();
-            harness.HttpManager.AddInstanceDiscoveryMockHandler(customPortAuthority);
+            harness.HttpManager.AddInstanceDiscoveryMockHandler(
+                customPortAuthority,
+                new Uri("https://login.microsoftonline.com/common/discovery/instance"));
 
             PublicClientApplication app = PublicClientApplicationBuilder.Create(TestConstants.ClientId)
                                                                         .WithAuthority(new Uri(customPortAuthority), false)
diff --git a/tests/Microsoft.Identity.Test.Unit/CoreTests/InstanceTests/InstanceDiscoveryManagerTests.cs b/tests/Microsoft.Identity.Test.Unit/CoreTests/InstanceTests/InstanceDiscoveryManagerTests.cs
index 485ab0f5b5..65d3d8b3ef 100644
--- a/tests/Microsoft.Identity.Test.Unit/CoreTests/InstanceTests/InstanceDiscoveryManagerTests.cs
+++ b/tests/Microsoft.Identity.Test.Unit/CoreTests/InstanceTests/InstanceDiscoveryManagerTests.cs
@@ -91,6 +91,51 @@ await ValidateSelfEntryAsync(new Uri(TestConstants.ADFSAuthority))
                 .ConfigureAwait(false);
         }
 
+        [TestMethod]
+        public async Task NetworkMetadataProvider_PreservesCustomPort_WhenDiscoveryHostMatchesAuthority_Async()
+        {
+            // Arrange
+            const string authority = "https://login.microsoftonline.com:5215/common/";
+            const string expectedDiscoveryEndpoint = "https://login.microsoftonline.com:5215/common/discovery/instance";
+            var networkMetadataProvider = new NetworkMetadataProvider(
+                _harness.HttpManager,
+                _networkCacheMetadataProvider);
+            var handler = _harness.HttpManager.AddInstanceDiscoveryMockHandler(
+                authority,
+                new Uri(expectedDiscoveryEndpoint));
+
+            // Act
+            await networkMetadataProvider.GetMetadataAsync(new Uri(authority), _testRequestContext)
+                .ConfigureAwait(false);
+
+            // Assert
+            Assert.IsNotNull(handler.ActualRequestMessage);
+            Assert.AreEqual(5215, handler.ActualRequestMessage.RequestUri.Port);
+            Assert.AreEqual(expectedDiscoveryEndpoint, handler.ActualRequestMessage.RequestUri.GetLeftPart(UriPartial.Path));
+        }
+
+        [TestMethod]
+        public async Task NetworkMetadataProvider_DropsCustomPort_WhenDiscoveryEndpointIsNotSetAndAuthorityIsUnknown_Async()
+        {
+            // Arrange
+            const string authority = "https://localhost:5215/common/";
+            const string expectedDiscoveryEndpoint = "https://login.microsoftonline.com/common/discovery/instance";
+            var networkMetadataProvider = new NetworkMetadataProvider(
+                _harness.HttpManager,
+                _networkCacheMetadataProvider);
+            var handler = _harness.HttpManager.AddInstanceDiscoveryMockHandler(authority);
+
+            // Act
+            await networkMetadataProvider.GetMetadataAsync(new Uri(authority), _testRequestContext)
+                .ConfigureAwait(false);
+
+            // Assert
+            Assert.IsNotNull(handler.ActualRequestMessage);
+            Assert.AreEqual("login.microsoftonline.com", handler.ActualRequestMessage.RequestUri.Host);
+            Assert.AreEqual(443, handler.ActualRequestMessage.RequestUri.Port);
+            Assert.AreEqual(expectedDiscoveryEndpoint, handler.ActualRequestMessage.RequestUri.GetLeftPart(UriPartial.Path));
+        }
+
         [TestMethod]
         public async Task NetworkCacheProvider_IsUsedFirst_Async()
         {