diff --git a/configure.ac b/configure.ac
index 001cf2ced..0433a7ccd 100755
--- a/configure.ac
+++ b/configure.ac
@@ -1206,6 +1206,9 @@ if test "$ipv6" = yes; then
 #include <ws2tcpip.h>
 #else
 #include <netinet/in.h>
+#if defined (__TANDEM)
+# include <netinet/in6.h>
+#endif
 #endif] ,
   struct sockaddr_in6 s; s.sin6_scope_id = 0; , have_sin6_scope_id=yes)
   if test "$have_sin6_scope_id" = yes; then
@@ -3354,6 +3357,7 @@ AC_CHECK_HEADERS(
         arpa/inet.h \
         net/if.h \
         netinet/in.h \
+	netinet/in6.h \
         sys/un.h \
         linux/tcp.h \
         netinet/tcp.h \
@@ -3404,6 +3408,9 @@ dnl default includes
 #ifdef HAVE_NETINET_IN_H
 #include <netinet/in.h>
 #endif
+#ifdef HAVE_NETINET_IN6_H
+#include <netinet/in6.h>
+#endif
 #ifdef HAVE_SYS_UN_H
 #include <sys/un.h>
 #endif
diff --git a/lib/curl_addrinfo.c b/lib/curl_addrinfo.c
index 6eb28bbcb..ec76f7540 100644
--- a/lib/curl_addrinfo.c
+++ b/lib/curl_addrinfo.c
@@ -27,6 +27,9 @@
 #ifdef HAVE_NETINET_IN_H
 #  include <netinet/in.h>
 #endif
+#ifdef HAVE_NETINET_IN6_H
+#  include <netinet/in6.h>
+#endif
 #ifdef HAVE_NETDB_H
 #  include <netdb.h>
 #endif
diff --git a/lib/hostcheck.c b/lib/hostcheck.c
index 23dc3d2a7..37bcc12c1 100644
--- a/lib/hostcheck.c
+++ b/lib/hostcheck.c
@@ -31,6 +31,9 @@
 #ifdef HAVE_NETINET_IN_H
 #include <netinet/in.h>
 #endif
+#ifdef HAVE_NETINET_IN6_H
+#include <netinet/in6.h>
+#endif
 
 #include "hostcheck.h"
 #include "strcase.h"
diff --git a/lib/hostip.c b/lib/hostip.c
index 7f010a037..ee2d4d2b5 100644
--- a/lib/hostip.c
+++ b/lib/hostip.c
@@ -25,6 +25,9 @@
 #ifdef HAVE_NETINET_IN_H
 #include <netinet/in.h>
 #endif
+#ifdef HAVE_NETINET_IN6_H
+#include <netinet/in6.h>
+#endif
 #ifdef HAVE_NETDB_H
 #include <netdb.h>
 #endif
diff --git a/lib/urldata.h b/lib/urldata.h
index 19cb6cafd..3152a11d9 100644
--- a/lib/urldata.h
+++ b/lib/urldata.h
@@ -85,6 +85,9 @@
 #ifdef HAVE_NETINET_IN_H
 #include <netinet/in.h>
 #endif
+#ifdef HAVE_NETINET_IN6_H
+#include <netinet/in6.h>
+#endif
 
 #include "timeval.h"