diff --git a/source/Uthernet2.cpp b/source/Uthernet2.cpp
index 7399beb8..dba2c61b 100644
--- a/source/Uthernet2.cpp
+++ b/source/Uthernet2.cpp
@@ -1407,7 +1407,7 @@ BYTE Uthernet2::IO_C0(WORD programcounter, WORD address, BYTE write, BYTE value,
     const uint16_t oldAddress = myDataAddress;
 #endif
 
-    const uint8_t loc = address & 0x0F;
+    const uint8_t loc = address & U2_C0X_MASK;
 
     if (write)
     {
diff --git a/source/W5100.h b/source/W5100.h
index 169fec93..7d8cec8d 100644
--- a/source/W5100.h
+++ b/source/W5100.h
@@ -2,10 +2,13 @@
 
 // Uthernet II registers
 
-#define U2_C0X_MODE_REGISTER      0x04
-#define U2_C0X_ADDRESS_HIGH       0x05
-#define U2_C0X_ADDRESS_LOW        0x06
-#define U2_C0X_DATA_PORT          0x07
+// only A0 and A1 are decoded
+#define U2_C0X_MASK               0x03
+
+#define U2_C0X_MODE_REGISTER      (0x04 & U2_C0X_MASK)
+#define U2_C0X_ADDRESS_HIGH       (0x05 & U2_C0X_MASK)
+#define U2_C0X_ADDRESS_LOW        (0x06 & U2_C0X_MASK)
+#define U2_C0X_DATA_PORT          (0x07 & U2_C0X_MASK)
 
 // W5100 registers and values