diff --git a/nmap_winconfig.h b/nmap_winconfig.h
index 6bf980a85..df15a4445 100644
--- a/nmap_winconfig.h
+++ b/nmap_winconfig.h
@@ -142,5 +142,8 @@
 #define DNET_INCLUDED 1
 #define PCRE_INCLUDED 1
 
+/* WinPCAP doesn't have this */
+#define PCAP_NETMASK_UNKNOWN 0
+
 #endif /* NMAP_WINCONFIG_H */
 
diff --git a/nsock/include/nsock_winconfig.h b/nsock/include/nsock_winconfig.h
index 5d77527cb..0f281c8cb 100644
--- a/nsock/include/nsock_winconfig.h
+++ b/nsock/include/nsock_winconfig.h
@@ -95,6 +95,8 @@
 
 #ifndef DISABLE_NSOCK_PCAP
 #define HAVE_PCAP 1
+/* WinPCAP doesn't have this */
+#define PCAP_NETMASK_UNKNOWN 0
 #endif
 
  /* WSAPoll() isn't available before Vista */