diff --git a/ci/test-03-forbidden.pl b/ci/test-03-forbidden.pl
index c42c6672..d2db1331 100755
--- a/ci/test-03-forbidden.pl
+++ b/ci/test-03-forbidden.pl
@@ -38,11 +38,11 @@ END
 $cmd7->stdout_is_eq("");
 $cmd7->stderr_is_eq("fping: specify only one of c, l\n");
 
-# fping -b 65509
-my $cmd8 = Test::Command->new(cmd => "fping -b 65509 127.0.0.1");
+# fping -b 65508
+my $cmd8 = Test::Command->new(cmd => "fping -b 65508 127.0.0.1");
 $cmd8->exit_is_num(1);
 $cmd8->stdout_is_eq("");
-$cmd8->stderr_is_eq("fping: data size 65509 not valid, must be lower than 65488\n");
+$cmd8->stderr_is_eq("fping: data size 65508 not valid, must not be larger than 65507\n");
 
 # fping -B 0.9
 my $cmd9 = Test::Command->new(cmd => "fping -B 0.9 127.0.0.1");
diff --git a/src/fping.c b/src/fping.c
index 696a017b..ae5986ff 100644
--- a/src/fping.c
+++ b/src/fping.c
@@ -134,8 +134,8 @@ extern int h_errno;
 
 /*** Ping packet defines ***/
 
-#define MAX_IP_PACKET 65536 /* (theoretical) max IP packet size */
-#define SIZE_IP_HDR 40
+#define MAX_IP_PACKET 65535 /* (theoretical) max IPv4 packet size */
+#define SIZE_IP_HDR 20 /* min IPv4 header size */
 #define SIZE_ICMP_HDR 8 /* from ip_icmp.h */
 #define MAX_PING_DATA (MAX_IP_PACKET - SIZE_IP_HDR - SIZE_ICMP_HDR)
 
@@ -952,7 +952,7 @@ int main(int argc, char **argv)
 #endif
 
     if (ping_data_size > MAX_PING_DATA) {
-        fprintf(stderr, "%s: data size %u not valid, must be lower than %u\n",
+        fprintf(stderr, "%s: data size %u not valid, must not be larger than %u\n",
             prog, ping_data_size, (unsigned int)MAX_PING_DATA);
         exit(1);
     }