i suspect it's part of an IPv4 prefix.
ip/rarpd.c has
	if(myipaddr(myip, net) < 0)
but
	uchar	myip[4];
yet myipaddr now sets IPaddrlen (16) bytes
and static char *efmt is allocated close to myip
in data.

if that becomes
	uchar	myip[IPaddrlen];
and the subsequent
		memmove(rp->spa, myip, sizeof(rp->spa));
becomes
		v6tov4(rp->spa, myip);
it might work, but then again
that might just be the first of several changes required.

i haven't got appropriate devices here to test rarpd