diff --git a/src/core/ipv4/dhcp.c b/src/core/ipv4/dhcp.c
index 80d3c3ea..5e62b2af 100644
--- a/src/core/ipv4/dhcp.c
+++ b/src/core/ipv4/dhcp.c
@@ -673,9 +673,9 @@ dhcp_handle_ack(struct netif *netif, struct dhcp_msg *msg_in)
   if (dhcp_option_given(dhcp, DHCP_OPTION_IDX_SUBNET_MASK)) {
     /* remember given subnet mask */
     ip4_addr_set_u32(&dhcp->offered_sn_mask, lwip_htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_SUBNET_MASK)));
-    dhcp->subnet_mask_given = 1;
+    dhcp->flags |= DHCP_FLAG_SUBNET_MASK_GIVEN;
   } else {
-    dhcp->subnet_mask_given = 0;
+    dhcp->flags &= ~DHCP_FLAG_SUBNET_MASK_GIVEN;
   }
 
   /* gateway router */
@@ -1088,7 +1088,7 @@ dhcp_bind(struct netif *netif)
     dhcp->t1_timeout = 0;
   }
 
-  if (dhcp->subnet_mask_given) {
+  if (dhcp->flags & DHCP_FLAG_SUBNET_MASK_GIVEN) {
     /* copy offered network mask */
     ip4_addr_copy(sn_mask, dhcp->offered_sn_mask);
   } else {
diff --git a/src/include/lwip/dhcp.h b/src/include/lwip/dhcp.h
index 49c266e8..1a3bba01 100644
--- a/src/include/lwip/dhcp.h
+++ b/src/include/lwip/dhcp.h
@@ -62,6 +62,8 @@ extern "C" {
 
 #define DHCP_BOOT_FILE_LEN      128U
 
+#define DHCP_FLAG_SUBNET_MASK_GIVEN 0x01
+
 /* AutoIP cooperation flags (struct dhcp.autoip_coop_state) */
 typedef enum {
   DHCP_AUTOIP_COOP_STATE_OFF  = 0,
@@ -78,8 +80,8 @@ struct dhcp
   u8_t state;
   /** retries of current request */
   u8_t tries;
-
-  u8_t subnet_mask_given;
+  /** see DHCP_FLAG_* */
+  u8_t flags;
 
   u16_t request_timeout; /* #ticks with period DHCP_FINE_TIMER_SECS for request timeout */
   u16_t t1_timeout;  /* #ticks with period DHCP_COARSE_TIMER_SECS for renewal time */