diff --git a/src/include/lwip/netif.h b/src/include/lwip/netif.h
index bde006a7..81dd338b 100644
--- a/src/include/lwip/netif.h
+++ b/src/include/lwip/netif.h
@@ -171,7 +171,7 @@ struct netif {
 #define NETIF_INIT_SNMP(netif, type, speed) \
   /* use "snmp_ifType" enum from snmp.h for "type", snmp_ifType_ethernet_csmacd by example */ \
   netif->link_type = type;    \
-  /* your link speed here */  \
+  /* your link speed here (units: bits per second) */  \
   netif->link_speed = speed;  \
   netif->ts = 0;              \
   netif->ifinoctets = 0;      \
diff --git a/src/netif/ethernetif.c b/src/netif/ethernetif.c
index b69bc4f6..26d2f818 100644
--- a/src/netif/ethernetif.c
+++ b/src/netif/ethernetif.c
@@ -288,7 +288,11 @@ ethernetif_init(struct netif *netif)
   netif->hostname = "lwip";
 #endif /* LWIP_NETIF_HOSTNAME */
 
-  /* initialize the snmp variables and counters inside the struct netif */
+  /*
+   * Initialize the snmp variables and counters inside the struct netif.
+   * The last argument should be replaced with your link speed, in units
+   * of bits per second.
+   */
   NETIF_INIT_SNMP(netif, snmp_ifType_ethernet_csmacd, ???);
 
   netif->state = ethernetif;