Time-To-Live

The -ttl <value> option sets the IPv4 packets time-to-live. The usefulness of this is in mapping paths through networks and determining ACL's on firewalls (setting the ttl to one past the packet filter can help to determine information about the filtering rules themselves). Repeated Nmap scans to a single port using differing ttl values will emulate a traceroute style network path map (Try it, its great fun for a while, until you get bored and realise traceroute does it all for you automatically!).