Add Socket::IPAddress#private?

spec/std/socket/address_spec.cr

@@ -118,6 +118,18 @@ describe Socket::IPAddress do
     Socket::IPAddress.new(Socket::IPAddress::UNSPECIFIED, 0).unspecified?.should be_true
     Socket::IPAddress.new(Socket::IPAddress::UNSPECIFIED6, 0).unspecified?.should be_true
   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
 end
 
 describe Socket::UNIXAddress do

src/socket/address.cr

@@ -219,7 +219,7 @@ class Socket
     # `127.0.0.0/24`. In IPv6 `::1` is the loopback address.
     def loopback? : Bool
       if addr = @addr4
-        addr.s_addr & 0x00000000ff_u32 == 0x0000007f_u32
+        addr.s_addr & 0x000000ff_u32 == 0x0000007f_u32
       elsif addr = @addr6
         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]
       else
@@ -238,6 +238,22 @@ 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
+    # and IPv6 Unique Local Addresses in fc00::/7 as defined in RFC 4193 are considered private.
+    def private? : Bool
+      if addr = @addr4
+        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
+      elsif addr = @addr6
+        ipv6_addr8(addr)[0] & 0xfe_u8 == 0xfc_u8
+      else
+        raise "unreachable!"
+      end
+    end
+
     private def ipv6_addr8(addr : LibC::In6Addr)
       {% if flag?(:darwin) || flag?(:openbsd) || flag?(:freebsd) || flag?(:dragonfly) %}
         addr.__u6_addr.__u6_addr8