diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index 0a0c9ea2..0a768448 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -1315,6 +1315,7 @@ do_dns_found(const char *name, struct ip_addr *ipaddr, void *arg)
   struct dns_api_msg *msg = (struct dns_api_msg*)arg;
 
   LWIP_ASSERT("DNS response for wrong host name", strcmp(msg->name, name) == 0);
+  LWIP_UNUSED_ARG(name);
 
   if (ipaddr == NULL) {
     /* timeout or memory error */
diff --git a/src/api/netdb.c b/src/api/netdb.c
index 69a331be..88135a55 100644
--- a/src/api/netdb.c
+++ b/src/api/netdb.c
@@ -315,7 +315,7 @@ lwip_getaddrinfo(const char *nodename, const char *servname,
   sa->sin_addr.s_addr = addr.addr;
   sa->sin_family = AF_INET;
   sa->sin_len = sizeof(struct sockaddr_in);
-  sa->sin_port = htons(port_nr);
+  sa->sin_port = htons((u16_t)port_nr);
 
   /* set up addrinfo */
   ai->ai_family = AF_INET;
diff --git a/src/api/sockets.c b/src/api/sockets.c
index 32c3752b..2ff41de0 100644
--- a/src/api/sockets.c
+++ b/src/api/sockets.c
@@ -318,7 +318,7 @@ lwip_accept(int s, struct sockaddr *addr, socklen_t *addrlen)
    * In that case, newconn->socket is counted down (newconn->socket--),
    * so nsock->rcvevent is >= 1 here!
    */
-  nsock->rcvevent += -1 - newconn->socket;
+  nsock->rcvevent += (s16_t)(-1 - newconn->socket);
   newconn->socket = newsock;
   sys_sem_signal(socksem);
 
@@ -455,14 +455,9 @@ lwip_listen(int s, int backlog)
     return -1;
 
   /* limit the "backlog" parameter to fit in an u8_t */
-  if (backlog < 0) {
-    backlog = 0;
-  }
-  if (backlog > 0xff) {
-    backlog = 0xff;
-  }
+  backlog = LWIP_MIN(LWIP_MAX(backlog, 0), 0xff);
 
-  err = netconn_listen_with_backlog(sock->conn, backlog);
+  err = netconn_listen_with_backlog(sock->conn, (u8_t)backlog);
 
   if (err != ERR_OK) {
     LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_listen(%d) failed, err=%d\n", s, err));
@@ -1557,7 +1552,7 @@ int
 lwip_setsockopt(int s, int level, int optname, const void *optval, socklen_t optlen)
 {
   struct lwip_socket *sock = get_socket(s);
-  int err = ERR_OK;
+  err_t err = ERR_OK;
   struct lwip_setgetsockopt_data data;
 
   if (!sock)
@@ -1901,21 +1896,21 @@ lwip_setsockopt_internal(void *arg)
   case IPPROTO_UDPLITE:
     switch (optname) {
     case UDPLITE_SEND_CSCOV:
-      if ((*(int*)optval != 0) && (*(int*)optval < 8)) {
+      if ((*(int*)optval != 0) && ((*(int*)optval < 8)) || (*(int*)optval > 0xffff)) {
         /* don't allow illegal values! */
         sock->conn->pcb.udp->chksum_len_tx = 8;
       } else {
-        sock->conn->pcb.udp->chksum_len_tx = *(int*)optval;
+        sock->conn->pcb.udp->chksum_len_tx = (u16_t)*(int*)optval;
       }
       LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_setsockopt(%d, IPPROTO_UDPLITE, UDPLITE_SEND_CSCOV) -> %d\n",
                   s, (*(int*)optval)) );
       break;
     case UDPLITE_RECV_CSCOV:
-      if ((*(int*)optval != 0) && (*(int*)optval < 8)) {
+      if ((*(int*)optval != 0) && ((*(int*)optval < 8)) || (*(int*)optval > 0xffff)) {
         /* don't allow illegal values! */
         sock->conn->pcb.udp->chksum_len_rx = 8;
       } else {
-        sock->conn->pcb.udp->chksum_len_rx = *(int*)optval;
+        sock->conn->pcb.udp->chksum_len_rx = (u16_t)*(int*)optval;
       }
       LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_setsockopt(%d, IPPROTO_UDPLITE, UDPLITE_RECV_CSCOV) -> %d\n",
                   s, (*(int*)optval)) );
diff --git a/src/core/dhcp.c b/src/core/dhcp.c
index 82068bfd..1f93d2fd 100644
--- a/src/core/dhcp.c
+++ b/src/core/dhcp.c
@@ -282,7 +282,9 @@ dhcp_select(struct netif *netif)
 #if LWIP_NETIF_HOSTNAME
     p = (const char*)netif->hostname;
     if (p != NULL) {
-      dhcp_option(dhcp, DHCP_OPTION_HOSTNAME, strlen(p));
+      u8_t namelen = (u8_t)strlen(p);
+      LWIP_ASSERT("DHCP: hostname is too long!", namelen < 255);
+      dhcp_option(dhcp, DHCP_OPTION_HOSTNAME, namelen);
       while (*p) {
         dhcp_option_byte(dhcp, *p++);
       }
@@ -990,7 +992,9 @@ dhcp_renew(struct netif *netif)
 #if LWIP_NETIF_HOSTNAME
     p = (const char*)netif->hostname;
     if (p != NULL) {
-      dhcp_option(dhcp, DHCP_OPTION_HOSTNAME, strlen(p));
+      u8_t namelen = (u8_t)strlen(p);
+      LWIP_ASSERT("DHCP: hostname is too long!", namelen < 255);
+      dhcp_option(dhcp, DHCP_OPTION_HOSTNAME, namelen);
       while (*p) {
         dhcp_option_byte(dhcp, *p++);
       }
@@ -1056,7 +1060,9 @@ dhcp_rebind(struct netif *netif)
 #if LWIP_NETIF_HOSTNAME
     p = (const char*)netif->hostname;
     if (p != NULL) {
-      dhcp_option(dhcp, DHCP_OPTION_HOSTNAME, strlen(p));
+      u8_t namelen = (u8_t)strlen(p);
+      LWIP_ASSERT("DHCP: hostname is too long!", namelen < 255);
+      dhcp_option(dhcp, DHCP_OPTION_HOSTNAME, namelen);
       while (*p) {
         dhcp_option_byte(dhcp, *p++);
       }
diff --git a/src/core/dns.c b/src/core/dns.c
index cb3d0a68..ac22e953 100644
--- a/src/core/dns.c
+++ b/src/core/dns.c
@@ -625,7 +625,7 @@ dns_send(u8_t numdns, const char* name, u8_t id)
     MEMCPY( query, &qry, SIZEOF_DNS_QUERY);
 
     /* resize pbuf to the exact dns query */
-    pbuf_realloc(p, (query + SIZEOF_DNS_QUERY) - ((char*)(p->payload)));
+    pbuf_realloc(p, (u16_t)((query + SIZEOF_DNS_QUERY) - ((char*)(p->payload))));
 
     /* connect to the server for faster receiving */
     udp_connect(dns_pcb, &dns_servers[numdns], DNS_SERVER_PORT);
@@ -741,12 +741,12 @@ dns_check_entries(void)
 static void
 dns_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16_t port)
 {
-  u8_t i;
+  u16_t i;
   char *pHostname;
   struct dns_hdr *hdr;
   struct dns_answer ans;
   struct dns_table_entry *pEntry;
-  u8_t nquestions, nanswers;
+  u16_t nquestions, nanswers;
 #if (DNS_USES_STATIC_BUF == 0)
   u8_t dns_payload[DNS_MSG_SIZE];
 #endif /* (DNS_USES_STATIC_BUF == 0) */
diff --git a/src/core/ipv4/inet_chksum.c b/src/core/ipv4/inet_chksum.c
index 185881ef..d7401229 100644
--- a/src/core/ipv4/inet_chksum.c
+++ b/src/core/ipv4/inet_chksum.c
@@ -181,7 +181,7 @@ lwip_standard_chksum(void *dataptr, int len)
     sum = SWAP_BYTES_IN_WORD(sum);
   }
 
-  return sum;
+  return (u16_t)sum;
 }
 #endif
 
@@ -263,7 +263,7 @@ lwip_standard_chksum(void *dataptr, int len)
     sum = SWAP_BYTES_IN_WORD(sum);
   }
 
-  return sum;
+  return (u16_t)sum;
 }
 #endif
 
diff --git a/src/core/ipv4/ip_frag.c b/src/core/ipv4/ip_frag.c
index 1939d831..852d0b9b 100644
--- a/src/core/ipv4/ip_frag.c
+++ b/src/core/ipv4/ip_frag.c
@@ -155,7 +155,8 @@ ip_reass_tmr(void)
 static int
 ip_reass_free_complete_datagram(struct ip_reassdata *ipr, struct ip_reassdata *prev)
 {
-  int pbufs_freed = 0;
+  u16_t pbufs_freed = 0;
+  u8_t clen;
   struct pbuf *p;
   struct ip_reass_helper *iprh;
 
@@ -175,7 +176,9 @@ ip_reass_free_complete_datagram(struct ip_reassdata *ipr, struct ip_reassdata *p
     /* Then, copy the original header into it. */
     SMEMCPY(p->payload, &ipr->iphdr, IP_HLEN);
     icmp_time_exceeded(p, ICMP_TE_FRAG);
-    pbufs_freed += pbuf_clen(p);
+    clen = pbuf_clen(p);
+    LWIP_ASSERT("pbufs_freed + clen <= 0xffff", pbufs_freed + clen <= 0xffff);
+    pbufs_freed += clen;
     pbuf_free(p);
   }
 #endif /* LWIP_ICMP */
@@ -189,8 +192,10 @@ ip_reass_free_complete_datagram(struct ip_reassdata *ipr, struct ip_reassdata *p
     pcur = p;
     /* get the next pointer before freeing */
     p = iprh->next_pbuf;
-    pbufs_freed += pbuf_clen(pcur);
-    pbuf_free(pcur);    
+    clen = pbuf_clen(pcur);
+    LWIP_ASSERT("pbufs_freed + clen <= 0xffff", pbufs_freed + clen <= 0xffff);
+    pbufs_freed += clen;
+    pbuf_free(pcur);
   }
   /* Then, unchain the struct ip_reassdata from the list and free it. */
   ip_reass_dequeue_datagram(ipr, prev);
diff --git a/src/core/snmp/msg_out.c b/src/core/snmp/msg_out.c
index b705aaca..c0557d70 100644
--- a/src/core/snmp/msg_out.c
+++ b/src/core/snmp/msg_out.c
@@ -234,10 +234,10 @@ snmp_send_trap(s8_t generic_trap, struct snmp_obj_id *eoid, s32_t specific_trap)
       /* lookup current source address for this dst */
       dst_if = ip_route(&td->dip);
       dst_ip.addr = ntohl(dst_if->ip_addr.addr);
-      trap_msg.sip_raw[0] = dst_ip.addr >> 24;
-      trap_msg.sip_raw[1] = dst_ip.addr >> 16;
-      trap_msg.sip_raw[2] = dst_ip.addr >> 8;
-      trap_msg.sip_raw[3] = dst_ip.addr;
+      trap_msg.sip_raw[0] = (u8_t)(dst_ip.addr >> 24);
+      trap_msg.sip_raw[1] = (u8_t)(dst_ip.addr >> 16);
+      trap_msg.sip_raw[2] = (u8_t)(dst_ip.addr >> 8);
+      trap_msg.sip_raw[3] = (u8_t)dst_ip.addr;
       trap_msg.gen_trap = generic_trap;
       trap_msg.spc_trap = specific_trap;
       if (generic_trap == SNMP_GENTRAP_ENTERPRISESPC)
diff --git a/src/core/tcp.c b/src/core/tcp.c
index a39857cf..a06a9864 100644
--- a/src/core/tcp.c
+++ b/src/core/tcp.c
@@ -422,7 +422,9 @@ u32_t tcp_update_rcv_ann_wnd(struct tcp_pcb *pcb)
       pcb->rcv_ann_wnd = 0;
     } else {
       /* keep the right edge of window constant */
-      pcb->rcv_ann_wnd = pcb->rcv_ann_right_edge - pcb->rcv_nxt;
+      u32_t new_rcv_ann_wnd = pcb->rcv_ann_right_edge - pcb->rcv_nxt;
+      LWIP_ASSERT("new_rcv_ann_wnd <= 0xffff", new_rcv_ann_wnd <= 0xffff);
+      pcb->rcv_ann_wnd = (u16_t)new_rcv_ann_wnd;
     }
     return 0;
   }
diff --git a/src/core/tcp_out.c b/src/core/tcp_out.c
index ddada420..726d4d60 100644
--- a/src/core/tcp_out.c
+++ b/src/core/tcp_out.c
@@ -60,7 +60,7 @@
 static void tcp_output_segment(struct tcp_seg *seg, struct tcp_pcb *pcb);
 
 static struct tcp_hdr *
-tcp_output_set_header(struct tcp_pcb *pcb, struct pbuf *p, int optlen,
+tcp_output_set_header(struct tcp_pcb *pcb, struct pbuf *p, u16_t optlen,
                       u32_t seqno_be /* already in network byte order */)
 {
   struct tcp_hdr *tcphdr = p->payload;
diff --git a/test/unit/tcp/tcp_helper.c b/test/unit/tcp/tcp_helper.c
index 98ee5eb6..092cdd19 100644
--- a/test/unit/tcp/tcp_helper.c
+++ b/test/unit/tcp/tcp_helper.c
@@ -57,7 +57,7 @@ tcp_create_segment(struct ip_addr* src_ip, struct ip_addr* dst_ip,
   struct pbuf* p;
   struct ip_hdr* iphdr;
   struct tcp_hdr* tcphdr;
-  u16_t pbuf_len = sizeof(struct ip_hdr) + sizeof(struct tcp_hdr) + data_len;
+  u16_t pbuf_len = (u16_t)(sizeof(struct ip_hdr) + sizeof(struct tcp_hdr) + data_len);
 
   p = pbuf_alloc(PBUF_RAW, pbuf_len, PBUF_POOL);
   EXPECT_RETNULL(p != NULL);
@@ -82,7 +82,7 @@ tcp_create_segment(struct ip_addr* src_ip, struct ip_addr* dst_ip,
   tcphdr->ackno = htonl(ackno);
   TCPH_HDRLEN_SET(tcphdr, sizeof(struct tcp_hdr)/4);
   TCPH_FLAGS_SET(tcphdr, headerflags);
-  tcphdr->wnd   = htonl(TCP_WND);
+  tcphdr->wnd   = htons(TCP_WND);
 
   /* copy data */
   memcpy((char*)tcphdr + sizeof(struct tcp_hdr), data, data_len);