Scanning Servers with Nmap

Nmap is a port scanner that can be used to determine whether a UDP or TCP port on a machine is open, and whether there is a server process accepting connections. Nmap can also find out if a firewall is protecting the machine scanned, and Nmap can scan whole networks. Let's scan the local client PC (which is obviously not protected by a firewall...):

opensuse01:~ # nmap 172.16.76.128
Starting nmap 3.81 ( http://www.insecure.org/nmap/ ) at 2005-12-02 18:02 CET
Interesting ports on localhost (172.16.76.128):
(The 1661 ports scanned but not shown below are in state: closed)
PORT   STATE SERVICE
22/tcp open  ssh
68/tcp open  dhcpclient
MAC Address: 00:0C:29:21:07:FC
Nmap finished: 1 IP address (1 host up) scanned in 1.773 seconds

There are two ports open on this system; port 1661 and other scanned ports are closed. If there were a firewall on this system, then scanning would not be that easy, because most firewalls detect scans and can prevent them. But there are many...