As far as I know, the W5100 does not support promiscous mode and an Arduino is probably not fast enough for even a 10Mbit/s network connection to scan in that mode (getting all packets and inspect them). Get a cheap router with OpenWRT (or a similar distro) and do it in Linux (netfilter/iptables or tcpdump). Most of the devices have a LED you can control with simple shell commands. If you need to control more than an LED later, you might go for a model with a USB (or TTL serial) port and you're able to attach an Arduino there.