net/netdev: Support multiple IPv6 addresses per device

Compatible with previous usage, because may network drivers are using old member name to print logs, and there's no significant need to change them now.

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

net/utils/net_mask2pref.c

@@ -207,4 +207,42 @@ uint8_t net_ipv6_mask2pref(FAR const uint16_t *mask)
   return preflen;
 }
 
+/****************************************************************************
+ * Name: net_ipv6_common_pref
+ *
+ * Description:
+ *   Calculate the common prefix length of two IPv6 addresses.
+ *
+ * Input Parameters:
+ *   a1,a2   Points to IPv6 addresses in the form of uint16_t[8]
+ *
+ * Returned Value:
+ *   The common prefix length, range 0-128 on success;  This function will
+ *   not fail.
+ *
+ ****************************************************************************/
+
+uint8_t net_ipv6_common_pref(FAR const uint16_t *a1, FAR const uint16_t *a2)
+{
+  uint8_t preflen;
+  int i;
+
+  /* Count the leading same 16-bit groups */
+
+  for (i = 0, preflen = 0; i < 8 && a1[i] == a2[i]; i++, preflen += 16);
+
+  /* Now i either, (1) indexes past the end of the mask, or (2) is the index
+   * to the first half-word that is not equal between the addresses.
+   */
+
+  if (i < 8)
+    {
+      preflen += net_msbits16(NTOHS(~(a1[i] ^ a2[i])));
+    }
+
+  /* Return the prefix length */
+
+  return preflen;
+}
+
 #endif /* CONFIG_NET_IPv6 */