diff --git a/src/core/inet.c b/src/core/inet.c
index 95c984d3..a34ea132 100644
--- a/src/core/inet.c
+++ b/src/core/inet.c
@@ -241,6 +241,7 @@ lwip_standard_chksum(void *dataptr, int len)
 /* inet_chksum_pseudo:
  *
  * Calculates the pseudo Internet checksum used by TCP and UDP for a pbuf chain.
+ * IP addresses are expected to be in network byte order.
  */
 
 u16_t
diff --git a/src/core/udp.c b/src/core/udp.c
index 5e1ef75b..6193961c 100644
--- a/src/core/udp.c
+++ b/src/core/udp.c
@@ -249,6 +249,8 @@ end:
  * @param dst_ip Destination IP address.
  * @param dst_port Destination UDP port.
  *
+ * dst_ip & dst_port are expected to be in the same byte order as in the pcb.
+ *
  * If the PCB already has a remote address association, it will
  * be restored after the data is sent.
  * 
@@ -430,6 +432,8 @@ udp_send(struct udp_pcb *pcb, struct pbuf *p)
  * bind to all local interfaces.
  * @param port local UDP port to bind with.
  *
+ * ipaddr & port are expected to be in the same byte order as in the pcb.
+ *
  * @return lwIP error code.
  * - ERR_OK. Successful. No error occured.
  * - ERR_USE. The specified ipaddr and port are already bound to by
@@ -528,6 +532,8 @@ udp_bind(struct udp_pcb *pcb, struct ip_addr *ipaddr, u16_t port)
  *
  * @return lwIP error code
  *
+ * ipaddr & port are expected to be in the same byte order as in the pcb.
+ *
  * @see udp_disconnect()
  */
 err_t
diff --git a/src/include/ipv4/lwip/ip.h b/src/include/ipv4/lwip/ip.h
index f5886740..866214d3 100644
--- a/src/include/ipv4/lwip/ip.h
+++ b/src/include/ipv4/lwip/ip.h
@@ -74,7 +74,9 @@ err_t ip_output_if(struct pbuf *p, struct ip_addr *src, struct ip_addr *dest,
    beginning of a PCB type definition. It is located here so that
    changes to this common part are made in one location instead of
    having to change all PCB structs. */
-#define IP_PCB struct ip_addr local_ip; \
+#define IP_PCB \
+  /* ip addresses in network byte order */ \
+  struct ip_addr local_ip; \
   struct ip_addr remote_ip; \
    /* Socket options */  \
   u16_t so_options;      \
diff --git a/src/include/lwip/sockets.h b/src/include/lwip/sockets.h
index 9a1b440b..b0523561 100644
--- a/src/include/lwip/sockets.h
+++ b/src/include/lwip/sockets.h
@@ -42,6 +42,7 @@
 extern "C" {
 #endif
 
+/* members are in network byte order */
 struct sockaddr_in {
   u8_t sin_len;
   u8_t sin_family;
diff --git a/src/include/lwip/tcp.h b/src/include/lwip/tcp.h
index 61be588d..24d245b1 100644
--- a/src/include/lwip/tcp.h
+++ b/src/include/lwip/tcp.h
@@ -170,6 +170,9 @@ void             tcp_rexmit_rto  (struct tcp_pcb *pcb);
 
 #define  TCP_MAXIDLE              TCP_KEEPCNT_DEFAULT * TCP_KEEPINTVL_DEFAULT  /* Maximum KEEPALIVE probe time */
 
+/* Fields are (of course) in network byte order.
+ * Some fields are converted to host byte order in tcp_input().
+ */
 #ifdef PACK_STRUCT_USE_INCLUDES
 #  include "arch/bpstruct.h"
 #endif
@@ -226,6 +229,7 @@ struct tcp_pcb {
   u8_t prio;
   void *callback_arg;
 
+  /* ports are in host byte order */
   u16_t local_port;
   u16_t remote_port;
   
@@ -238,6 +242,8 @@ struct tcp_pcb {
 #define TF_GOT_FIN   (u8_t)0x20U   /* Connection was closed by the remote end. */
 #define TF_NODELAY   (u8_t)0x40U   /* Disable Nagle algorithm */
 
+  /* the rest of the fields are in host byte order
+     as we have to do some math with them */
   /* receiver variables */
   u32_t rcv_nxt;   /* next seqno expected */
   u16_t rcv_wnd;   /* receiver window */
diff --git a/src/include/lwip/udp.h b/src/include/lwip/udp.h
index 089066e3..90d4d302 100644
--- a/src/include/lwip/udp.h
+++ b/src/include/lwip/udp.h
@@ -44,6 +44,7 @@ extern "C" {
 
 #define UDP_HLEN 8
 
+/* Fields are (of course) in network byte order. */
 struct udp_hdr {
   PACK_STRUCT_FIELD(u16_t src);
   PACK_STRUCT_FIELD(u16_t dest);  /* src/dest UDP ports */
@@ -64,10 +65,13 @@ struct udp_pcb {
   struct udp_pcb *next;
 
   u8_t flags;
+  /* ports are in host byte order */
   u16_t local_port, remote_port;
   
+  /* used for UDP_LITE only */
   u16_t chksum_len;
   
+  /* addr and port are in same byte order as in the pcb */
   void (* recv)(void *arg, struct udp_pcb *pcb, struct pbuf *p,
     struct ip_addr *addr, u16_t port);
   void *recv_arg;