Add IsIPv6UniqueLocal property (#48853)

* Add IsIPv6UniqueLocal property

Author: annchous

src/libraries/System.Net.Primitives/ref/System.Net.Primitives.cs

@@ -240,6 +240,7 @@ public IPAddress(System.ReadOnlySpan<byte> address, long scopeid) { }
         public bool IsIPv6Multicast { get { throw null; } }
         public bool IsIPv6SiteLocal { get { throw null; } }
         public bool IsIPv6Teredo { get { throw null; } }
+        public bool IsIPv6UniqueLocal { get { throw null; } }
         public long ScopeId { get { throw null; } set { } }
         public override bool Equals([System.Diagnostics.CodeAnalysis.NotNullWhen(true)] object? comparand) { throw null; }
         public byte[] GetAddressBytes() { throw null; }

src/libraries/System.Net.Primitives/src/System/Net/IPAddress.cs

@@ -484,6 +484,15 @@ public bool IsIPv6Teredo
             }
         }
 
+        /// <summary>Gets whether the address is an IPv6 Unique Local address.</summary>
+        public bool IsIPv6UniqueLocal
+        {
+            get
+            {
+                return IsIPv6 && ((_numbers![0] & 0xFE00) == 0xFC00);
+            }
+        }
+
         // 0:0:0:0:0:FFFF:x.x.x.x
         public bool IsIPv4MappedToIPv6
         {

src/libraries/System.Net.Primitives/tests/FunctionalTests/IPAddressTest.cs

@@ -233,6 +233,14 @@ public static void IsIPV6Teredo_Get_Success()
             Assert.False(IPV4Address1().IsIPv6Teredo);
         }
 
+        [Fact]
+        public static void IsIPv6UniqueLocal_Get_Success()
+        {
+            Assert.True(IPAddress.Parse("FC00::1").IsIPv6UniqueLocal);
+            Assert.False(IPAddress.Parse("Fe08::1").IsIPv6UniqueLocal);
+            Assert.False(IPV4Address1().IsIPv6UniqueLocal);
+        }
+
         [Fact]
         public static void Equals_Compare_Success()
         {