Key Unix networking tools and files

Tools

ifconfig: Get information about your various interfaces, as well as configure them.
iwconfig: Similar to ifconfig, but designed for wireless interfaces. We won't worry much about this one.
route: Manage your routing table.
iptables: Manage your firewall.
dhclient: Script to configure your networking settings from a DHCP server.
arp: manage your arp cache.
netstat: List information about currently used sockets, local and net-related.
tcpdump: The poor man's ethereal (then again, what's cheaper than free?). If you're having network problems and can't even download your preferred packet sniffer, this can be helpful for diagnosing problems.
etc/init.d/network: Network configuration tool. Runs when you boot up, but can be run again if you play with your configuration files.

/etc/resolv.conf: List of nameservers for DNS lookup.
/etc/sysconfig/networking/devices/ifcfg-eth* (Red Hat/Fedora specific): Configuration files for your different interfaces. These are what get read when the network system tool is run (modifying them requires a network restart to take effect).