ignore 0.0.0.0
https://sources.debian.org/patches/arpwatch/2.1a15-6/25ignore_zero_ip/

remove DECnet remnants

Index: db.c
--- db.c.orig
+++ db.c
@@ -95,6 +95,11 @@ ent_add(register u_int32_t a, register u_char *e, time
 	u_char *e2;
 	time_t t2;
 
+	/* Ignore 0.0.0.0 */
+	if (a == 0) {
+		return(1);
+	}
+
 	/* Lookup ip address */
 	ap = ainfo_find(a);
 
@@ -124,14 +129,11 @@ ent_add(register u_int32_t a, register u_char *e, time
 		ep = ap->elist[1];
 		if (MEMCMP(e, ep->e, 6) == 0) {
 			/*
-			 * Suppress report when less than
-			 * FLIPFLOP_DELTA and one of the two ethernet
-			 * addresses is a DECnet logical.
+			 * Suppress report when less than FLIPFLOP_DELTA.
 			 */
 			t2 = ap->elist[0]->t;
 			e2 = ap->elist[0]->e;
-			if (t - t2 < FLIPFLIP_DELTA &&
-			    (isdecnet(e) || isdecnet(e2)))
+			if (t - t2 < FLIPFLIP_DELTA)
 				dosyslog(LOG_INFO,
 				    "suppressed DECnet flip flop", a, e, e2);
 			else