add libip
diff --git a/src/libip/classmask.c b/src/libip/classmask.c
new file mode 100644
index 0000000..5932f87
--- /dev/null
+++ b/src/libip/classmask.c
@@ -0,0 +1,25 @@
+#include <u.h>
+#include <libc.h>
+#include <ip.h>
+
+uchar classmask[4][16] = {
+ 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0xff, 0xff,0x00,0x00,0x00,
+ 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0xff, 0xff,0x00,0x00,0x00,
+ 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0xff, 0xff,0xff,0x00,0x00,
+ 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0x00,
+};
+
+uchar*
+defmask(uchar *ip)
+{
+ return classmask[ip[IPv4off]>>6];
+}
+
+void
+maskip(uchar *from, uchar *mask, uchar *to)
+{
+ int i;
+
+ for(i = 0; i < IPaddrlen; i++)
+ to[i] = from[i] & mask[i];
+}
