diff --git a/lib/ipv4.h b/lib/ipv4.h
index 033116bb..49aa13a6 100644
--- a/lib/ipv4.h
+++ b/lib/ipv4.h
@@ -37,6 +37,7 @@ typedef u32 ip_addr;
 
 #define BITS_PER_IP_ADDRESS 32
 #define STD_ADDRESS_P_LENGTH 15
+#define SIZE_OF_IP_HEADER 24
 
 #define IPA_NONE (_MI(0))
 
diff --git a/lib/ipv6.h b/lib/ipv6.h
index 90aedb94..2efd9e69 100644
--- a/lib/ipv6.h
+++ b/lib/ipv6.h
@@ -24,6 +24,7 @@ typedef struct ipv6_addr {
 
 #define BITS_PER_IP_ADDRESS 128
 #define STD_ADDRESS_P_LENGTH 39
+#define SIZE_OF_IP_HEADER 40
 
 #define IPA_NONE _MI(0,0,0,0)
 
diff --git a/proto/ospf/ospf.h b/proto/ospf/ospf.h
index 128fb490..edb3c389 100644
--- a/proto/ospf/ospf.h
+++ b/proto/ospf/ospf.h
@@ -9,7 +9,7 @@
 #ifndef _BIRD_OSPF_H_
 #define _BIRD_OSPF_H_
 
-#define SIPH 64		/* FIXME Size Of IP header */
+#define SIPH SIZE_OF_IP_HEADER
 #define MAXNETS 10
 #ifdef LOCAL_DEBUG
 #define OSPF_FORCE_DEBUG 1