Footprinting

Network Enumeration is the process of identifying domain names and associated networks

In other words if myfavoritewebsite.com has registered public ip addresses what are they and how many are there

A simple whois search from popular dns resources on the web can tell a lot about a potential target
www.dnsstuff.com
www.samspade.org

both sites will give you a lot of usefull info on a domain such as
owner
owner phone number
address
email address – which is an interesting point I’ll make later in the notes
sometimes it will even give a list of domain servers

assuming all of this info is correct a potential attacker has now info that he/she can use to SE with

getting back to that email address we found earlier an attacker could use this for several purposes a SE aspect where an attacker could forge email using that address to gain more info and poteintally a login
if the organization uses the same standard for email logins as network logins there might be similarities there to possibly have a way into the internal network

Other Helpful Techniques Used In Footprinting
Ping Sweep: Ping a range of IP addresses to find out which machines are awake.
TCP Scans: Scan ports on machines to see which services are offered. TCP scans can be performed by scanning a single port on a range of IPs, or by scanning a range of ports on a single IP. Both techniques yeild helpful information.
UDP Scans: Send garbage UDP packets to a desired port. I normally don't perform UDP scans a whole lot because most machines respond with an ICMP 'port unreachable' message. Meaning that no service is available.
OS Indentification: This involves sending illegal ICMP or TCP packets to a machine
Nmap would probably be the most popular tool that is used in tcp scans udp scans and OS dectection .
Just remember that the goal of footprinting is to gain as much info as possible as discreetly as possible