How Spanning Tree Attacks Work 95 attempt to show you the network topology, per se, but it does show you flows in the network and does help you discover other hosts. Nmap is an open-source network-mapping tool. Nmap provides a GUI called Zenmap that can help visualize your network. Unlike EtherApe, you give Nmap a list of hosts to scan, and it builds its data by actively scanning the hosts (instead of passively sniffing packets). This allows you to detect not only what hosts are available, but in many cases what the hosts do, as Nmap gives you a list of inter- esting ports that are open on the host. The Zenmap GUI provides the "topology" view shown in Figure 5.12. The Nmap tool is described in Chapter 1, "Denial of Service." There are other utilities, as well, including commercial utilities like Lumeta's IPsonar AA and SolarWinds LANsurveyor, BB and freeware applications like LanToplog. CC