diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9a38bdbe..8b4be348 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -54,6 +54,7 @@ and this project adheres to
   disable DHCP.
 * Disabled waiting in `EthernetClient::close()` for altcp clients because it's
   not defined.
+* Changed configuration macro checks to use value instead of definedness.
 
 ### Fixed
 * Fixed a `printf` conversion specifier in the _RandomNumbers_ example. This was
diff --git a/src/QNEthernet.cpp b/src/QNEthernet.cpp
index 2d7065b2..afb54b47 100644
--- a/src/QNEthernet.cpp
+++ b/src/QNEthernet.cpp
@@ -533,7 +533,7 @@ bool EthernetClass::setMACAddressAllowed(const uint8_t mac[6],
   if (netif_ == nullptr) {
     return false;
   }
-#ifndef QNETHERNET_ENABLE_PROMISCUOUS_MODE
+#if !QNETHERNET_ENABLE_PROMISCUOUS_MODE
   return enet_set_mac_address_allowed(mac, flag);
 #else
   return flag;  // Can't disallow MAC addresses
diff --git a/src/QNEthernet.h b/src/QNEthernet.h
index c0532097..8d266130 100644
--- a/src/QNEthernet.h
+++ b/src/QNEthernet.h
@@ -94,7 +94,7 @@ class EthernetClass final {
 
   // Returns whether promiscuous mode is enabled.
   static constexpr bool isPromiscuousMode() {
-#ifdef QNETHERNET_ENABLE_PROMISCUOUS_MODE
+#if QNETHERNET_ENABLE_PROMISCUOUS_MODE
     return true;
 #else
     return false;
@@ -408,7 +408,7 @@ class EthernetClass final {
 // Instance for interacting with the library.
 STATIC_INIT_DECL(EthernetClass, Ethernet);
 
-#ifdef QNETHERNET_ENABLE_CUSTOM_WRITE
+#if QNETHERNET_ENABLE_CUSTOM_WRITE
 // stdout output.
 extern Print *stdoutPrint;
 
diff --git a/src/QNEthernetFrame.cpp b/src/QNEthernetFrame.cpp
index 7414efa3..a68705a1 100644
--- a/src/QNEthernetFrame.cpp
+++ b/src/QNEthernetFrame.cpp
@@ -4,7 +4,7 @@
 // QNEthernetFrame.cpp contains an EthernetFrame implementation.
 // This file is part of the QNEthernet library.
 
-#ifdef QNETHERNET_ENABLE_RAW_FRAME_SUPPORT
+#if QNETHERNET_ENABLE_RAW_FRAME_SUPPORT
 
 #include "QNEthernetFrame.h"
 
diff --git a/src/QNEthernetFrame.h b/src/QNEthernetFrame.h
index ec8e4b9a..95f71c86 100644
--- a/src/QNEthernetFrame.h
+++ b/src/QNEthernetFrame.h
@@ -7,7 +7,7 @@
 #ifndef QNETHERNET_ETHERNETFRAME_H_
 #define QNETHERNET_ETHERNETFRAME_H_
 
-#ifdef QNETHERNET_ENABLE_RAW_FRAME_SUPPORT
+#if QNETHERNET_ENABLE_RAW_FRAME_SUPPORT
 
 // C++ includes
 #include <cstddef>
diff --git a/src/arch/cc.h b/src/arch/cc.h
index 67499d96..82eba061 100644
--- a/src/arch/cc.h
+++ b/src/arch/cc.h
@@ -26,7 +26,7 @@
   } while (0)
 extern void qnethernet_stdio_flush(int file);
 
-#ifndef QNETHERNET_LWIP_MEMORY_IN_RAM1
+#if !QNETHERNET_LWIP_MEMORY_IN_RAM1
 #define LWIP_DECLARE_MEMORY_ALIGNED(variable_name, size) \
   u8_t variable_name[(size)] DMAMEM __attribute__((aligned(MEM_ALIGNMENT)))
 #else
diff --git a/src/default_altcp_functions.cpp b/src/default_altcp_functions.cpp
index 3bce60b1..6434e77b 100644
--- a/src/default_altcp_functions.cpp
+++ b/src/default_altcp_functions.cpp
@@ -7,7 +7,7 @@
 
 #include "lwip/opt.h"
 
-#if LWIP_ALTCP && defined(QNETHERNET_ENABLE_ALTCP_DEFAULT_FUNCTIONS)
+#if LWIP_ALTCP && QNETHERNET_ENABLE_ALTCP_DEFAULT_FUNCTIONS
 
 #include <cstdint>
 #include <functional>
@@ -31,4 +31,4 @@ __attribute__((weak))
 std::function<void(const altcp_allocator_t &)> qnethernet_altcp_free_allocator =
     [](const altcp_allocator_t &allocator) {};
 
-#endif  // LWIP_ALTCP && defined(QNETHERNET_ENABLE_ALTCP_DEFAULT_FUNCTIONS)
+#endif  // LWIP_ALTCP && QNETHERNET_ENABLE_ALTCP_DEFAULT_FUNCTIONS
diff --git a/src/lwip_driver.h b/src/lwip_driver.h
index 70af4605..2a982581 100644
--- a/src/lwip_driver.h
+++ b/src/lwip_driver.h
@@ -97,7 +97,7 @@ bool phy_link_is_crossover();
 // This adds any extra padding bytes given by ETH_PAD_SIZE.
 bool enet_output_frame(const uint8_t *frame, size_t len);
 
-#ifndef QNETHERNET_ENABLE_PROMISCUOUS_MODE
+#if !QNETHERNET_ENABLE_PROMISCUOUS_MODE
 
 // For joining and leaving multicast groups; these call
 // enet_set_mac_address_allowed() with the MAC addresses related to the given
diff --git a/src/lwip_hooks.h b/src/lwip_hooks.h
index a109483d..af79ffc8 100644
--- a/src/lwip_hooks.h
+++ b/src/lwip_hooks.h
@@ -11,7 +11,7 @@
 #include "lwip/netif.h"
 #include "lwip/pbuf.h"
 
-#ifdef QNETHERNET_ENABLE_RAW_FRAME_SUPPORT
+#if QNETHERNET_ENABLE_RAW_FRAME_SUPPORT
 
 #define LWIP_HOOK_UNKNOWN_ETH_PROTOCOL(p, netif) \
   unknown_eth_protocol((p), (netif))
diff --git a/src/lwip_t41.c b/src/lwip_t41.c
index 6d86d9ca..a23da5f8 100644
--- a/src/lwip_t41.c
+++ b/src/lwip_t41.c
@@ -173,7 +173,7 @@
 //   Payload(1500) + FCS(4)
 #define BUF_SIZE (((ETH_PAD_SIZE + 6 + 6 + 2 + 2 + 2 + 1500 + 4) + 63) & ~63)
 
-#ifndef QNETHERNET_BUFFERS_IN_RAM1
+#if !QNETHERNET_BUFFERS_IN_RAM1
 #define MULTIPLE_OF_32(x) (((x) + 31) & ~31)
 #define BUFFER_DMAMEM DMAMEM
 #else
@@ -639,7 +639,7 @@ static void low_level_init() {
              | ENET_RCR_PADEN      // Padding is removed
              | ENET_RCR_RMII_MODE
              | ENET_RCR_FCE        // Flow control enable
-#ifdef QNETHERNET_ENABLE_PROMISCUOUS_MODE
+#if QNETHERNET_ENABLE_PROMISCUOUS_MODE
              | ENET_RCR_PROM       // Promiscuous mode
 #endif  // QNETHERNET_ENABLE_PROMISCUOUS_MODE
              | ENET_RCR_MII_MODE;
@@ -761,7 +761,7 @@ static struct pbuf *low_level_input(volatile enetbufferdesc_t *pBD) {
   } else {
     p = pbuf_alloc(PBUF_RAW, pBD->length, PBUF_POOL);
     if (p) {
-#ifndef QNETHERNET_BUFFERS_IN_RAM1
+#if !QNETHERNET_BUFFERS_IN_RAM1
       arm_dcache_delete(pBD->buffer, MULTIPLE_OF_32(p->tot_len));
 #endif  // !QNETHERNET_BUFFERS_IN_RAM1
       pbuf_take(p, pBD->buffer, p->tot_len);
@@ -832,7 +832,7 @@ static err_t low_level_output(struct netif *netif, struct pbuf *p) {
     LINK_STATS_INC(link.drop);
     return ERR_BUF;
   }
-#ifndef QNETHERNET_BUFFERS_IN_RAM1
+#if !QNETHERNET_BUFFERS_IN_RAM1
   arm_dcache_flush_delete(pBD->buffer, MULTIPLE_OF_32(copied));
 #endif  // !QNETHERNET_BUFFERS_IN_RAM1
   update_bufdesc(pBD, copied);
@@ -952,7 +952,7 @@ static inline int check_link_status(int state) {
   return 0;
 }
 
-#if LWIP_IGMP && !defined(QNETHERNET_ENABLE_PROMISCUOUS_MODE)
+#if LWIP_IGMP && !QNETHERNET_ENABLE_PROMISCUOUS_MODE
 // Multicast filter for letting the hardware know which packets to let in.
 static err_t multicast_filter(struct netif *netif, const ip4_addr_t *group,
                               enum netif_mac_filter_action action) {
@@ -973,7 +973,7 @@ static err_t multicast_filter(struct netif *netif, const ip4_addr_t *group,
       // ERR_USE seems like the best fit of the choices
       // Next best seems to be ERR_IF
 }
-#endif  // LWIP_IGMP && !defined(QNETHERNET_ENABLE_PROMISCUOUS_MODE)
+#endif  // LWIP_IGMP && !QNETHERNET_ENABLE_PROMISCUOUS_MODE
 
 // --------------------------------------------------------------------------
 //  Public Interface
@@ -1013,7 +1013,7 @@ bool enet_has_hardware() {
   return (s_initState != kInitStateNoHardware);
 }
 
-#ifdef QNETHERNET_INTERNAL_END_STOPS_ALL
+#if QNETHERNET_INTERNAL_END_STOPS_ALL
 // Removes the current netif, if any.
 static void remove_netif() {
   if (s_isNetifAdded) {
@@ -1070,10 +1070,10 @@ bool enet_init(const uint8_t mac[ETH_HWADDR_LEN],
     autoip_set_struct(&s_netif, &s_autoip);
 #endif  // LWIP_AUTOIP
 
-#if LWIP_IGMP && !defined(QNETHERNET_ENABLE_PROMISCUOUS_MODE)
+#if LWIP_IGMP && !QNETHERNET_ENABLE_PROMISCUOUS_MODE
     // Multicast filtering, to allow desired multicast packets in
     netif_set_igmp_mac_filter(&s_netif, &multicast_filter);
-#endif  // LWIP_IGMP && !defined(QNETHERNET_ENABLE_PROMISCUOUS_MODE)
+#endif  // LWIP_IGMP && !QNETHERNET_ENABLE_PROMISCUOUS_MODE
   } else {
     // Just set the MAC address
 
@@ -1100,7 +1100,7 @@ void enet_deinit() {
   // is restarted after calling end(), so gate the following two blocks with a
   // macro for now
 
-#ifdef QNETHERNET_INTERNAL_END_STOPS_ALL
+#if QNETHERNET_INTERNAL_END_STOPS_ALL
   remove_netif();  // TODO: This also causes issues (see notes in enet_init())
 
   if (s_initState == kInitStateInitialized) {
@@ -1208,7 +1208,7 @@ bool enet_output_frame(const uint8_t *frame, size_t len) {
   volatile enetbufferdesc_t *pBD = get_bufdesc();
 
   memcpy((uint8_t *)pBD->buffer + ETH_PAD_SIZE, frame, len);
-#ifndef QNETHERNET_BUFFERS_IN_RAM1
+#if !QNETHERNET_BUFFERS_IN_RAM1
   arm_dcache_flush_delete(pBD->buffer, MULTIPLE_OF_32(len + ETH_PAD_SIZE));
 #endif  // !QNETHERNET_BUFFERS_IN_RAM1
   update_bufdesc(pBD, len + ETH_PAD_SIZE);
@@ -1220,7 +1220,7 @@ bool enet_output_frame(const uint8_t *frame, size_t len) {
 //  MAC Address Filtering
 // --------------------------------------------------------------------------
 
-#ifndef QNETHERNET_ENABLE_PROMISCUOUS_MODE
+#if !QNETHERNET_ENABLE_PROMISCUOUS_MODE
 
 // CRC-32 routine for computing the 4-byte FCS for multicast lookup.
 static uint32_t crc32(uint32_t crc, const uint8_t *data, size_t len) {
diff --git a/src/lwip_unsupported.c b/src/lwip_unsupported.c
index 72d5a00f..45ff4c12 100644
--- a/src/lwip_unsupported.c
+++ b/src/lwip_unsupported.c
@@ -57,7 +57,7 @@ bool enet_output_frame(const uint8_t *frame, size_t len) {
 //  MAC Address Filtering
 // --------------------------------------------------------------------------
 
-#ifndef QNETHERNET_ENABLE_PROMISCUOUS_MODE
+#if !QNETHERNET_ENABLE_PROMISCUOUS_MODE
 
 bool enet_set_mac_address_allowed(const uint8_t *mac, bool allow) {
   return false;
diff --git a/src/security/RandomDevice.cpp b/src/security/RandomDevice.cpp
index 011a80fa..20fc7361 100644
--- a/src/security/RandomDevice.cpp
+++ b/src/security/RandomDevice.cpp
@@ -8,7 +8,7 @@
 
 #include <pgmspace.h>
 
-#if defined(__IMXRT1062__) && !defined(QNETHERNET_USE_ENTROPY_LIB)
+#if defined(__IMXRT1062__) && !QNETHERNET_USE_ENTROPY_LIB
 
 #include "entropy.h"
 
diff --git a/src/security/entropy.c b/src/security/entropy.c
index f520d4e3..b3036978 100644
--- a/src/security/entropy.c
+++ b/src/security/entropy.c
@@ -5,7 +5,7 @@
 // This file is part of the QNEthernet library.
 
 #if defined(__IMXRT1062__)
-#if !defined(QNETHERNET_USE_ENTROPY_LIB)
+#if !QNETHERNET_USE_ENTROPY_LIB
 
 #include "entropy.h"
 
diff --git a/src/security/entropy.h b/src/security/entropy.h
index d2fae856..f5ae4908 100644
--- a/src/security/entropy.h
+++ b/src/security/entropy.h
@@ -8,7 +8,7 @@
 #define QNETHERNET_SECURITY_ENTROPY_H_
 
 #if defined(__IMXRT1062__)
-#if !defined(QNETHERNET_USE_ENTROPY_LIB)
+#if !QNETHERNET_USE_ENTROPY_LIB
 
 #ifdef __cplusplus
 extern "C" {
diff --git a/src/sys_arch.cpp b/src/sys_arch.cpp
index 3e70ca49..5324fdd6 100644
--- a/src/sys_arch.cpp
+++ b/src/sys_arch.cpp
@@ -8,7 +8,7 @@
 #include <unistd.h>
 
 // C++ includes
-#ifdef QNETHERNET_ENABLE_CUSTOM_WRITE
+#if QNETHERNET_ENABLE_CUSTOM_WRITE
 #include <cerrno>
 #endif  // QNETHERNET_ENABLE_CUSTOM_WRITE
 #include <cstdint>
@@ -72,7 +72,7 @@ const char *lwip_strerr(err_t err) {
 //  stdio
 // --------------------------------------------------------------------------
 
-#ifdef QNETHERNET_ENABLE_CUSTOM_WRITE
+#if QNETHERNET_ENABLE_CUSTOM_WRITE
 
 // The user program can set these to something initialized. For example,
 // `&Serial`, after `Serial.begin(speed)`.
@@ -96,7 +96,7 @@ extern "C" {
 // Gets the Print* for the given file descriptor.
 static inline Print *getPrint(int file) {
   switch (file) {
-#ifdef QNETHERNET_ENABLE_CUSTOM_WRITE
+#if QNETHERNET_ENABLE_CUSTOM_WRITE
     case STDOUT_FILENO:
       return ::qindesign::network::stdoutPrint;
     case STDERR_FILENO:
@@ -113,7 +113,7 @@ static inline Print *getPrint(int file) {
   }
 }
 
-#ifdef QNETHERNET_ENABLE_CUSTOM_WRITE
+#if QNETHERNET_ENABLE_CUSTOM_WRITE
 
 // Define this function to provide expanded stdio output behaviour.
 // See: https://forum.pjrc.com/threads/28473-Quick-Guide-Using-printf()-on-Teensy-ARM