diff --git a/src/ip.h b/src/ip.h
index 6d5273d..b67d42a 100644
--- a/src/ip.h
+++ b/src/ip.h
@@ -7,6 +7,12 @@ struct ip_common {
     uint8_t proto;
 };
 
+static inline uint8_t
+ip_get_version(const uint8_t *data)
+{
+    return data[0] >> 4;
+}
+
 static inline uint16_t
 ip_read16(const uint8_t *src)
 {
@@ -21,7 +27,7 @@ ip_get_common(struct ip_common *ic, const uint8_t *data, size_t size)
     if (size < 20)
         return 1;
 
-    switch (data[0] >> 4) {
+    switch (ip_get_version(data)) {
     case 4:
         ic->tc = data[1];
         ic->proto = data[9];
diff --git a/src/tun.c b/src/tun.c
index 64153eb..65ea125 100644
--- a/src/tun.c
+++ b/src/tun.c
@@ -224,7 +224,7 @@ tun_write(int fd, const void *data, size_t size)
 #ifdef GT_BSD_TUN
     uint32_t family;
 
-    switch (ip_get_version(data, size)) {
+    switch (ip_get_version(data)) {
     case 4:
         family = htonl(AF_INET);
         break;