diff --git a/qse/include/qse/si/SocketAddress.hpp b/qse/include/qse/si/SocketAddress.hpp
index aced48bb..5ee16e30 100644
--- a/qse/include/qse/si/SocketAddress.hpp
+++ b/qse/include/qse/si/SocketAddress.hpp
@@ -96,6 +96,11 @@ public:
 	// TODO: isLinkLocal() const QSE_CPP_NOEXCEPT
 	// TODO: isSiteLocal() const QSE_CPP_NOEXCEPT
 	// TODO: isV4Mapped() const QSE_CPP_NOEXCEPT
+	//
+	//TODO: bool isInIpSubnet (const qse_ip4ad_t* addr, int prefix) const  QSE_CPP_NOEXCEPT;
+	//TODO: bool isInIpSubnet (const qse_ip6ad_t* addr, int prefix) const  QSE_CPP_NOEXCEPT;
+	bool isInIpSubnet (const qse_nwad_t* addr, int prefix) const  QSE_CPP_NOEXCEPT;
+	//TODO: bool isInIpSubnet (const qse_skad_t* addr, int prefix) const  QSE_CPP_NOEXCEPT;
 
 	qse_mchar_t* toStrBuf (qse_mchar_t* buf, qse_size_t len) const QSE_CPP_NOEXCEPT;
 	qse_wchar_t* toStrBuf (qse_wchar_t* buf, qse_size_t len) const QSE_CPP_NOEXCEPT;
diff --git a/qse/include/qse/si/nwad.h b/qse/include/qse/si/nwad.h
index 349452ba..2f0af6e8 100644
--- a/qse/include/qse/si/nwad.h
+++ b/qse/include/qse/si/nwad.h
@@ -248,6 +248,15 @@ QSE_EXPORT qse_uint16_t qse_getnwadport (
 	qse_nwad_t*   nwad
 );
 
+/**
+ * The qse_getnwadipsize() function returns the size of the contained
+ * IP address. If the given \a nwad doesn't represent  an IP address,
+ * it returns 0.
+ */
+QSE_EXPORT int qse_getnwadipsize (
+	qse_nwad_t* nwad
+);
+
 /**
  * The qse_mbstonwad() function converts a multi-byte string \a mbs to a 
  * network address and stores it to memory pointed to by \a nwad.
diff --git a/qse/lib/si/SocketAddress.cpp b/qse/lib/si/SocketAddress.cpp
index d00c0f0f..7fcb743b 100644
--- a/qse/lib/si/SocketAddress.cpp
+++ b/qse/lib/si/SocketAddress.cpp
@@ -393,6 +393,49 @@ bool SocketAddress::isLoopBack () const QSE_CPP_NOEXCEPT
 	return false;
 }
 
+bool SocketAddress::isInIpSubnet (const qse_nwad_t* addr, int prefix) const QSE_CPP_NOEXCEPT
+{
+	switch (addr->type)
+	{
+		case QSE_NWAD_IN4:
+			if (prefix >= 1 && prefix <= 32)
+			{
+				const qse_ip4ad_t* me = this->getIp4addr();
+				if (me)
+				{
+					qse_ip4ad_t mask;
+					qse_prefixtoip4ad (prefix, &mask);
+					return (me->value & mask.value) == (addr->u.in4.addr.value & mask.value);
+				}
+			}
+			break;
+
+		case QSE_NWAD_IN6:
+			if (prefix >= 1 && prefix <= 128)
+			{
+				const qse_ip6ad_t* me = this->getIp6addr();
+				if (me)
+				{
+					qse_ip6ad_t mask;
+					qse_prefixtoip6ad (prefix, &mask);
+					const qse_uint32_t* x = (const qse_uint32_t*)me->value;
+					const qse_uint32_t* y = (const qse_uint32_t*)addr->u.in6.addr.value;
+					const qse_uint32_t* z = (const qse_uint32_t*)mask.value;
+					return (x[0] & z[0]) == (y[0] & z[0]) &&
+					       (x[1] & z[1]) == (y[1] & z[1]) &&
+					       (x[2] & z[2]) == (y[2] & z[2]) &&
+					       (x[3] & z[3]) == (y[3] & z[3]);
+				}
+			}
+			break;
+
+		default: 
+			break;
+	}
+
+	return false;
+}
+
 qse_wchar_t* SocketAddress::toStrBuf (qse_wchar_t* buf, qse_size_t len) const QSE_CPP_NOEXCEPT
 {
 	qse_nwad_t nwad;
diff --git a/qse/lib/si/nwad.c b/qse/lib/si/nwad.c
index a670a8fb..0f5a14a5 100644
--- a/qse/lib/si/nwad.c
+++ b/qse/lib/si/nwad.c
@@ -35,7 +35,7 @@
 #include "../cmn/mem-prv.h"
 
 
-void qse_initnwadwithip4ad (qse_nwad_t* nwad, qse_uint16_t port,	const qse_ip4ad_t* ipad)
+void qse_initnwadwithip4ad (qse_nwad_t* nwad, qse_uint16_t port, const qse_ip4ad_t* ipad)
 {
 	QSE_MEMSET (nwad, 0, QSE_SIZEOF(*nwad));
 	nwad->type = QSE_NWAD_IN4;
@@ -116,6 +116,22 @@ qse_uint16_t qse_getnwadport (qse_nwad_t* nwad)
 	}
 }
 
+int qse_getnwadipsize (qse_nwad_t* nwad)
+{
+	switch (nwad->type)
+	{
+		case QSE_NWAD_IN4:
+			return QSE_SIZEOF(qse_ip4ad_t);
+
+		case QSE_NWAD_IN6:
+			return QSE_SIZEOF(qse_ip6ad_t);
+
+		case QSE_NWAD_LOCAL:
+		default:
+			return 0;
+	}
+}
+
 int qse_mbstonwad (const qse_mchar_t* str, qse_nwad_t* nwad)
 {
 	return qse_mbsntonwad (str, qse_mbslen(str), nwad);
diff --git a/qse/lib/si/sck.c b/qse/lib/si/sck.c
index 104f6296..22a490c8 100644
--- a/qse/lib/si/sck.c
+++ b/qse/lib/si/sck.c
@@ -294,6 +294,7 @@ int qse_fini_sck_conn (qse_sck_hnd_t handle)
 	}
 	else if (ret != 0) 
 	{
+		errno = ret;
 		return -1; /* failure */
 	}