diff --git a/bitcoin-qt.pro b/bitcoin-qt.pro
index beb3c5b4..92103f1e 100644
--- a/bitcoin-qt.pro
+++ b/bitcoin-qt.pro
@@ -2,7 +2,7 @@ TEMPLATE = app
 TARGET = bitcoin-qt
 VERSION = 0.6.99
 INCLUDEPATH += src src/json src/qt
-DEFINES += QT_GUI BOOST_THREAD_USE_LIB BOOST_SPIRIT_THREADSAFE USE_IPV6
+DEFINES += QT_GUI BOOST_THREAD_USE_LIB BOOST_SPIRIT_THREADSAFE
 CONFIG += no_include_pwd
 
 # for boost 1.37, add -mt to the boost libraries
@@ -71,6 +71,18 @@ contains(FIRST_CLASS_MESSAGING, 1) {
     DEFINES += FIRST_CLASS_MESSAGING
 }
 
+# use: qmake "USE_IPV6=1" ( enabled by default; default)
+#  or: qmake "USE_IPV6=0" (disabled by default)
+#  or: qmake "USE_IPV6=-" (not supported)
+contains(USE_IPV6, -) {
+	message(Building without IPv6 support)
+} else {
+	count(USE_IPV6, 0) {
+		USE_IPV6=1
+	}
+	DEFINES += USE_IPV6=$$USE_IPV6
+}
+
 contains(BITCOIN_NEED_QT_PLUGINS, 1) {
     DEFINES += BITCOIN_NEED_QT_PLUGINS
     QTPLUGIN += qcncodecs qjpcodecs qtwcodecs qkrcodecs qtaccessiblewidgets
diff --git a/src/init.cpp b/src/init.cpp
index 1a1e31c2..25756c4e 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -500,6 +500,12 @@ bool AppInit2()
                 SetLimited(net);
         }
     }
+#if defined(USE_IPV6)
+#if ! USE_IPV6
+    else
+        SetLimited(NET_IPV6);
+#endif
+#endif
 
     CService addrProxy;
     bool fProxy = false;
diff --git a/src/makefile.linux-mingw b/src/makefile.linux-mingw
index e22a9f38..ad74aa6c 100644
--- a/src/makefile.linux-mingw
+++ b/src/makefile.linux-mingw
@@ -5,6 +5,7 @@
 DEPSDIR:=/usr/i586-mingw32msvc
 
 USE_UPNP:=0
+USE_IPV6:=1
 
 INCLUDEPATHS= \
  -I"$(CURDIR)" \
@@ -28,18 +29,25 @@ LIBS= \
  -l ssl \
  -l crypto
 
-DEFS=-D_MT -DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE -DUSE_IPV6
+DEFS=-D_MT -DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE
 DEBUGFLAGS=-g
 CFLAGS=-O2 -w -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter $(DEBUGFLAGS) $(DEFS) $(INCLUDEPATHS)
 
 TESTDEFS = -DTEST_DATA_DIR=$(abspath test/data)
 
-ifdef USE_UPNP
+ifndef USE_UPNP
+	override USE_UPNP = -
+endif
+ifneq (${USE_UPNP}, -)
 	LIBPATHS += -L"$(DEPSDIR)/miniupnpc"
 	LIBS += -l miniupnpc -l iphlpapi
 	DEFS += -DSTATICLIB -DUSE_UPNP=$(USE_UPNP)
 endif
 
+ifneq (${USE_IPV6}, -)
+	DEFS += -DUSE_IPV6=$(USE_IPV6)
+endif
+
 LIBS += -l mingwthrd -l kernel32 -l user32 -l gdi32 -l comdlg32 -l winspool -l winmm -l shell32 -l comctl32 -l ole32 -l oleaut32 -l uuid -l rpcrt4 -l advapi32 -l ws2_32 -l mswsock -l shlwapi
 
 # TODO: make the mingw builds smarter about dependencies, like the linux/osx builds are
diff --git a/src/makefile.mingw b/src/makefile.mingw
index 74897656..6102dad6 100644
--- a/src/makefile.mingw
+++ b/src/makefile.mingw
@@ -3,6 +3,7 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 USE_UPNP:=0
+USE_IPV6:=1
 
 INCLUDEPATHS= \
  -I"C:\boost-1.49.0-mgw" \
@@ -23,19 +24,26 @@ LIBS= \
  -l ssl \
  -l crypto
 
-DEFS=-DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE -DUSE_IPV6
+DEFS=-DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE
 DEBUGFLAGS=-g
 CFLAGS=-mthreads -O2 -w -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter $(DEBUGFLAGS) $(DEFS) $(INCLUDEPATHS)
 
 TESTDEFS = -DTEST_DATA_DIR=$(abspath test/data)
 
-ifdef USE_UPNP
+ifndef USE_UPNP
+	override USE_UPNP = -
+endif
+ifneq (${USE_UPNP}, -)
  INCLUDEPATHS += -I"C:\miniupnpc-1.6-mgw"
  LIBPATHS += -L"C:\miniupnpc-1.6-mgw"
  LIBS += -l miniupnpc -l iphlpapi
  DEFS += -DSTATICLIB -DUSE_UPNP=$(USE_UPNP)
 endif
 
+ifneq (${USE_IPV6}, -)
+	DEFS += -DUSE_IPV6=$(USE_IPV6)
+endif
+
 LIBS += -l kernel32 -l user32 -l gdi32 -l comdlg32 -l winspool -l winmm -l shell32 -l comctl32 -l ole32 -l oleaut32 -l uuid -l rpcrt4 -l advapi32 -l ws2_32 -l mswsock -l shlwapi
 
 # TODO: make the mingw builds smarter about dependencies, like the linux/osx builds are
diff --git a/src/makefile.osx b/src/makefile.osx
index 97787839..de829b9e 100644
--- a/src/makefile.osx
+++ b/src/makefile.osx
@@ -20,6 +20,7 @@ LIBPATHS= \
  -L"$(DEPSDIR)/lib/db48"
 
 USE_UPNP:=1
+USE_IPV6:=1
 
 LIBS= -dead_strip
 
@@ -53,7 +54,7 @@ LIBS += \
 TESTDEFS += -DBOOST_TEST_DYN_LINK
 endif
 
-DEFS=-DMAC_OSX -DMSG_NOSIGNAL=0 -DBOOST_SPIRIT_THREADSAFE -DUSE_IPV6
+DEFS=-DMAC_OSX -DMSG_NOSIGNAL=0 -DBOOST_SPIRIT_THREADSAFE
 
 ifdef RELEASE
 # Compile for maximum compatibility and smallest size.
@@ -96,7 +97,10 @@ OBJS= \
     obj/walletdb.o \
     obj/noui.o
 
-ifdef USE_UPNP
+ifndef USE_UPNP
+	override USE_UPNP = -
+endif
+ifneq (${USE_UPNP}, -)
 	DEFS += -DUSE_UPNP=$(USE_UPNP)
 ifdef STATIC
 	LIBS += $(DEPSDIR)/lib/libminiupnpc.a
@@ -105,6 +109,10 @@ else
 endif
 endif
 
+ifneq (${USE_IPV6}, -)
+	DEFS += -DUSE_IPV6=$(USE_IPV6)
+endif
+
 all: bitcoind
 
 # auto-generated dependencies:
diff --git a/src/makefile.unix b/src/makefile.unix
index c9a09ad0..ac42743d 100644
--- a/src/makefile.unix
+++ b/src/makefile.unix
@@ -3,10 +3,11 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 USE_UPNP:=0
+USE_IPV6:=1
 
 LINK:=$(CXX)
 
-DEFS=-DUSE_IPV6 -DBOOST_SPIRIT_THREADSAFE
+DEFS=-DBOOST_SPIRIT_THREADSAFE
 
 DEFS += $(addprefix -I,$(CURDIR) $(CURDIR)/obj $(BOOST_INCLUDE_PATH) $(BDB_INCLUDE_PATH) $(OPENSSL_INCLUDE_PATH))
 LIBS = $(addprefix -L,$(BOOST_LIB_PATH) $(BDB_LIB_PATH) $(OPENSSL_LIB_PATH))
@@ -43,6 +44,10 @@ ifneq (${USE_UPNP}, -)
 	DEFS += -DUSE_UPNP=$(USE_UPNP)
 endif
 
+ifneq (${USE_IPV6}, -)
+	DEFS += -DUSE_IPV6=$(USE_IPV6)
+endif
+
 LIBS+= \
  -Wl,-B$(LMODE2) \
    -l z \