firewall

If you have been using the Internet for any length of time, and especially if you work at a larger company and browse the Web while you are at work, you have probably heard the term firewall used. For example, you often hear people in companies say things like, "I can''t use that site because they won''t let it through the firewall."
If you have a fast Internet connection into your home (either a DSL connection or a cable modem), you may have found yourself hearing about firewalls for your home network as well. It turns out that a small home network has many of the same security issues that a large corporate network does. You can use a firewall to protect your home network and family from offensive Web sites and potential hackers.
Basically, a firewall is a barrier to keep destructive forces away from your property. In fact, that''s why its called a firewall. Its job is similar to a physical firewall that keeps a fire from spreading from one area to the next
A firewall is simply a program or hardware device that filters the information coming through the Internet connection into your private network or computer system. If an incoming packet of information is flagged by the filters, it is not allowed through.
If you have read the article How Web Servers Work, then you know a good bit about how data moves on the Internet, and you can easily see how a firewall helps protect computers inside a large company. Let''s say that you work at a company with 500 employees. The company will therefore have hundreds of computers that all have network cards connecting them together. In addition, the company will have one or more connections to the Internet through something like T1 or T3 lines. Without a firewall in place, all of those hundreds of computers are directly accessible to anyone on the Internet. A person who knows what he or she is doing can probe those computers, try to make FTP connections to them, try to make telnet connections to them and so on. If one employee makes a mistake and leaves a security hole, hackers can get to the machine and exploit the hole. .
Firewalls use one or more of three methods to control traffic flowing in and out of the network:
Packet filtering - Packets (small chunks of data) are analyzed against a set of filters. Packets that make it through the filters are sent to the requesting system and all others are discarded.
Proxy service - Information from the Internet is retrieved by the firewall and then sent to the requesting system and vice versa.
Stateful inspection - A newer method that doesn''t examine the contents of each packet but instead compares certain key parts of the packet to a database of trusted information. Information traveling from inside the firewall to the outside is monitored for specific defining characteristics, then incoming information is compared to these characteristics. If the comparison yields a reasonable match, the information is allowed through. Otherwise it is discarded.
IP addresses - Each machine on the Internet is assigned a unique address called an IP address. IP addresses are 32-bit numbers, normally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this: 216.27.61.137. For example, if a certain IP address outside the company is reading too many files from a server, the firewall can block all traffic to or from that IP address.
Domain names - Because it is hard to remember the string of numbers that make up an IP address, and because IP addresses sometimes need to change, all servers on the Internet also have human-readable names, called domain names. For example, it is easier for most of us to remember www.howstuffworks.com than it is to remember 216.27.61.137. A company might block all access to certain domain names, or allow access only to specific domain names.
Protocols - The Protocol is the pre-defined way that someone who wants to use a service talks with that service. The "someone" could be a person, but more often it is a computer program like a Web browser. Protocols are often text, and simply describe how the client and server will have their conversation. The http in the Web''s protocol. Some common protocols that you can set firewall filters for include:
IP (Internet Protocol) - the main delivery system for information over the Internet
TCP (Transmission Control Protocol) - used to break apart and rebuild information that travels over the Internet
HTTP (Hyper Text Transfer Protocol) - used for Web pages
FTP (File Transfer Protocol) - used to download and upload files
UDP (User Datagram Protocol) - used for information that requires no response, such as streaming audio and video
ICMP (Internet Control Message Protocol) - used by a router to exchange the information with other routers
SMTP (Simple Mail Transport Protocol) - used to send text-based information (e-mail)
SNMP (Simple Network Management Protocol) - used to collect system information from a remote computer
Telnet - used to perform commands on a remote computer