diff --git a/src/core/def.c b/src/core/def.c
index bdece2f9..70d3e580 100644
--- a/src/core/def.c
+++ b/src/core/def.c
@@ -101,7 +101,7 @@ char*
 lwip_strnstr(const char* buffer, const char* token, size_t n)
 {
   const char* p;
-  int tokenlen = (int)strlen(token);
+  size_t tokenlen = strlen(token);
   if (tokenlen == 0) {
     return LWIP_CONST_CAST(char *, buffer);
   }
diff --git a/src/core/ipv4/icmp.c b/src/core/ipv4/icmp.c
index 6f5311d0..e030ecc9 100644
--- a/src/core/ipv4/icmp.c
+++ b/src/core/ipv4/icmp.c
@@ -81,7 +81,7 @@ icmp_input(struct pbuf *p, struct netif *inp)
 #endif /* LWIP_DEBUG */
   struct icmp_echo_hdr *iecho;
   const struct ip_hdr *iphdr_in;
-  s16_t hlen;
+  u16_t hlen;
   const ip4_addr_t* src;
 
   ICMP_STATS_INC(icmp.recv);
@@ -148,7 +148,7 @@ icmp_input(struct pbuf *p, struct netif *inp)
     }
 #endif
 #if LWIP_ICMP_ECHO_CHECK_INPUT_PBUF_LEN
-    if (pbuf_header(p, (hlen + PBUF_LINK_HLEN + PBUF_LINK_ENCAPSULATION_HLEN))) {
+    if (pbuf_header(p, (s16_t)(hlen + PBUF_LINK_HLEN + PBUF_LINK_ENCAPSULATION_HLEN))) {
       /* p is not big enough to contain link headers
        * allocate a new one and copy p into it
        */
@@ -167,7 +167,7 @@ icmp_input(struct pbuf *p, struct netif *inp)
       /* copy the ip header */
       MEMCPY(r->payload, iphdr_in, hlen);
       /* switch r->payload back to icmp header (cannot fail) */
-      if (pbuf_header(r, -hlen)) {
+      if (pbuf_header(r, (s16_t)-hlen)) {
         LWIP_ASSERT("icmp_input: moving r->payload to icmp header failed\n", 0);
         pbuf_free(r);
         goto icmperr;
@@ -194,7 +194,7 @@ icmp_input(struct pbuf *p, struct netif *inp)
     /* We generate an answer by switching the dest and src ip addresses,
      * setting the icmp type to ECHO_RESPONSE and updating the checksum. */
     iecho = (struct icmp_echo_hdr *)p->payload;
-    if (pbuf_header(p, hlen)) {
+    if (pbuf_header(p, (s16_t)hlen)) {
       LWIP_DEBUGF(ICMP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("Can't move over header in packet"));
     } else {
       err_t ret;
diff --git a/src/core/ipv4/ip4_addr.c b/src/core/ipv4/ip4_addr.c
index eb812afb..2d479923 100644
--- a/src/core/ipv4/ip4_addr.c
+++ b/src/core/ipv4/ip4_addr.c
@@ -183,10 +183,10 @@ ip4addr_aton(const char *cp, ip4_addr_t *addr)
     }
     for (;;) {
       if (isdigit(c)) {
-        val = (val * base) + (int)(c - '0');
+        val = (val * base) + (u32_t)(c - '0');
         c = *++cp;
       } else if (base == 16 && isxdigit(c)) {
-        val = (val << 4) | (int)(c + 10 - (islower(c) ? 'a' : 'A'));
+        val = (val << 4) | (u32_t)(c + 10 - (islower(c) ? 'a' : 'A'));
         c = *++cp;
       } else {
         break;
@@ -310,7 +310,7 @@ ip4addr_ntoa_r(const ip4_addr_t *addr, char *buf, int buflen)
     do {
       rem = *ap % (u8_t)10;
       *ap /= (u8_t)10;
-      inv[i++] = '0' + rem;
+      inv[i++] = (char)('0' + rem);
     } while (*ap);
     while (i--) {
       if (len++ >= buflen) {
diff --git a/src/core/ipv6/ip6.c b/src/core/ipv6/ip6.c
index f17b6c85..a542a869 100644
--- a/src/core/ipv6/ip6.c
+++ b/src/core/ipv6/ip6.c
@@ -715,7 +715,7 @@ netif_found:
 options_done:
 
   /* p points to IPv6 header again. */
-  pbuf_header_force(p, ip_data.current_ip_header_tot_len);
+  pbuf_header_force(p, (s16_t)ip_data.current_ip_header_tot_len);
 
   /* send to upper layers */
   LWIP_DEBUGF(IP6_DEBUG, ("ip6_input: \n"));
diff --git a/src/core/ipv6/ip6_addr.c b/src/core/ipv6/ip6_addr.c
index 964291ee..aa06659a 100644
--- a/src/core/ipv6/ip6_addr.c
+++ b/src/core/ipv6/ip6_addr.c
@@ -132,8 +132,8 @@ ip6addr_aton(const char *cp, ip6_addr_t *addr)
     } else if (isxdigit(*s)) {
       /* add current digit */
       current_block_value = (current_block_value << 4) +
-          (isdigit(*s) ? *s - '0' :
-          10 + (islower(*s) ? *s - 'a' : *s - 'A'));
+          (isdigit(*s) ? (u32_t)(*s - '0') :
+          (u32_t)(10 + (islower(*s) ? *s - 'a' : *s - 'A')));
     } else {
       /* unexpected digit, space? CRLF? */
       break;
diff --git a/src/core/netif.c b/src/core/netif.c
index f7211f9a..1d3de0ff 100644
--- a/src/core/netif.c
+++ b/src/core/netif.c
@@ -478,7 +478,7 @@ netif_find(const char *name)
     return NULL;
   }
 
-  num = name[2] - '0';
+  num = (u8_t)(name[2] - '0');
 
   for (netif = netif_list; netif != NULL; netif = netif->next) {
     if (num == netif->num &&
diff --git a/src/core/pbuf.c b/src/core/pbuf.c
index 52f088d2..60477383 100644
--- a/src/core/pbuf.c
+++ b/src/core/pbuf.c
@@ -568,11 +568,11 @@ pbuf_header_impl(struct pbuf *p, s16_t header_size_increment, u8_t force)
   }
 
   if (header_size_increment < 0) {
-    increment_magnitude = -header_size_increment;
+    increment_magnitude = (u16_t)-header_size_increment;
     /* Check that we aren't going to move off the end of the pbuf */
     LWIP_ERROR("increment_magnitude <= p->len", (increment_magnitude <= p->len), return 1;);
   } else {
-    increment_magnitude = header_size_increment;
+    increment_magnitude = (u16_t)header_size_increment;
 #if 0
     /* Can't assert these as some callers speculatively call
          pbuf_header() to see if it's OK.  Will return 1 below instead. */
diff --git a/src/core/udp.c b/src/core/udp.c
index 52830c4f..a9164149 100644
--- a/src/core/udp.c
+++ b/src/core/udp.c
@@ -404,7 +404,7 @@ udp_input(struct pbuf *p, struct netif *inp)
          destination address was broadcast/multicast. */
       if (!broadcast && !ip_addr_ismulticast(ip_current_dest_addr())) {
         /* move payload pointer back to ip header */
-        pbuf_header_force(p, ip_current_header_tot_len() + UDP_HLEN);
+        pbuf_header_force(p, (s16_t)(ip_current_header_tot_len() + UDP_HLEN));
         icmp_port_unreach(ip_current_is_v6(), p);
       }
 #endif /* LWIP_ICMP || LWIP_ICMP6 */
diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index 0965e09b..174f9b79 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -1363,7 +1363,7 @@
  * for an additional encapsulation header before ethernet headers (e.g. 802.11)
  */
 #if !defined PBUF_LINK_ENCAPSULATION_HLEN || defined __DOXYGEN__
-#define PBUF_LINK_ENCAPSULATION_HLEN    0
+#define PBUF_LINK_ENCAPSULATION_HLEN    0u
 #endif
 
 /**
diff --git a/src/include/lwip/prot/tcp.h b/src/include/lwip/prot/tcp.h
index c2c03aa4..67fe7b9e 100644
--- a/src/include/lwip/prot/tcp.h
+++ b/src/include/lwip/prot/tcp.h
@@ -85,7 +85,7 @@ PACK_STRUCT_END
 
 #define TCPH_HDRLEN_SET(phdr, len) (phdr)->_hdrlen_rsvd_flags = lwip_htons(((len) << 12) | TCPH_FLAGS(phdr))
 #define TCPH_FLAGS_SET(phdr, flags) (phdr)->_hdrlen_rsvd_flags = (((phdr)->_hdrlen_rsvd_flags & PP_HTONS(~TCP_FLAGS)) | lwip_htons(flags))
-#define TCPH_HDRLEN_FLAGS_SET(phdr, len, flags) (phdr)->_hdrlen_rsvd_flags = lwip_htons(((len) << 12) | (flags))
+#define TCPH_HDRLEN_FLAGS_SET(phdr, len, flags) (phdr)->_hdrlen_rsvd_flags = (u16_t)(lwip_htons((u16_t)((len) << 12) | (flags)))
 
 #define TCPH_SET_FLAG(phdr, flags ) (phdr)->_hdrlen_rsvd_flags = ((phdr)->_hdrlen_rsvd_flags | lwip_htons(flags))
 #define TCPH_UNSET_FLAG(phdr, flags) (phdr)->_hdrlen_rsvd_flags = ((phdr)->_hdrlen_rsvd_flags & ~lwip_htons(flags))