Add Socket::IPAddress#private? (#9517)

Co-authored-by: Brian J. Cardiff <bcardiff@gmail.com>

spec/std/socket/address_spec.cr

@@ -128,6 +128,18 @@ describe Socket::IPAddress do
     Socket::IPAddress.valid_port?(65_536).should be_false
   end
 
+  it "#private?" do
+    Socket::IPAddress.new("192.168.0.1", 0).private?.should be_true
+    Socket::IPAddress.new("192.100.0.1", 0).private?.should be_false
+    Socket::IPAddress.new("172.16.0.1", 0).private?.should be_true
+    Socket::IPAddress.new("172.10.0.1", 0).private?.should be_false
+    Socket::IPAddress.new("10.0.0.1", 0).private?.should be_true
+    Socket::IPAddress.new("1.1.1.1", 0).private?.should be_false
+    Socket::IPAddress.new("fd00::1", 0).private?.should be_true
+    Socket::IPAddress.new("fb00::1", 0).private?.should be_false
+    Socket::IPAddress.new("2001:4860:4860::8888", 0).private?.should be_false
+  end
+
   it "#==" do
     Socket::IPAddress.new("127.0.0.1", 8080).should eq Socket::IPAddress.new("127.0.0.1", 8080)
     Socket::IPAddress.new("127.0.0.1", 8080).hash.should eq Socket::IPAddress.new("127.0.0.1", 8080).hash

src/socket/address.cr

@@ -208,7 +208,7 @@ class Socket
     def loopback? : Bool
       case addr = @addr
       in LibC::InAddr
-        addr.s_addr & 0x00000000ff_u32 == 0x0000007f_u32
+        addr.s_addr & 0x000000ff_u32 == 0x0000007f_u32
       in LibC::In6Addr
         ipv6_addr8(addr) == StaticArray[0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 0_u8, 1_u8]
       end
@@ -224,6 +224,21 @@ class Socket
       end
     end
 
+    # Returns `true` if this IP is a private address.
+    #
+    # IPv4 addresses in `10.0.0.0/8`, `172.16.0.0/12` and `192.168.0.0/16` as defined in [RFC 1918](https://tools.ietf.org/html/rfc1918)
+    # and IPv6 Unique Local Addresses in `fc00::/7` as defined in [RFC 4193](https://tools.ietf.org/html/rfc4193) are considered private.
+    def private? : Bool
+      case addr = @addr
+      in LibC::InAddr
+        addr.s_addr & 0x000000ff_u32 == 0x00000000a_u32 ||     # 10.0.0.0/8
+          addr.s_addr & 0x000000f0ff_u32 == 0x0000010ac_u32 || # 172.16.0.0/12
+          addr.s_addr & 0x000000ffff_u32 == 0x0000a8c0_u32     # 192.168.0.0/16
+      in LibC::In6Addr
+        ipv6_addr8(addr)[0] & 0xfe_u8 == 0xfc_u8
+      end
+    end
+
     private def ipv6_addr8(addr : LibC::In6Addr)
       {% if flag?(:darwin) || flag?(:bsd) %}
         addr.__u6_addr.__u6_addr8