A firewall provides security by controlling access between a network and an untrusted network. A firewall can be a hardware device, software, or combination of hardware and software.
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How a Firewall Provides Network Security

A firewall provides security by controlling access between a network and an untrusted network. A firewall can allow or block traffic from entering or exiting a network. A firewall can be a hardware device, software, or combination of hardware and software.

Although a firewall can be used to control traffic between parts of an Intranet or between parts of networks belonging to different companies, firewalls are usually used to control traffic between a private network and the Internet. No responsible network administrator would connect their network to the Internet without a firewall.

- One benefit of a firewall is that it proves a single point of administration for providing network traffic security.

- A firewall is good at keeping intruders out of a network, but a user within the network can purposely download data that contains a virus, so a firewall cannot protect against all viruses.

There are four types of firewalls.

* Packet-filter
* Circuit-layer
* Application-layer
* Stateful Inspection

Proxy Server

A Proxy Server is a host that multiple computers on a LAN connect to in order to get access to an outside network, such as the Internet. The Proxy Server presents only its own single IP address to the outside network, thus acting as a proxy for the computers on the LAN. The process of changing the individual IP addresses of computers on the LAN to one IP address is called Network Address Translation (NAT).

Because a Proxy Server hides individual IP addresses on the LAN, it performs some function as a firewall, but a proxy server does not block access to the network. A firewall blocks access and it can provide the proxy function by performing NAT. Other network devices, like routers, perform fire wall functionality because they provide packet-filtering capabilities.

Packet-Filter Firewall

The first firewall technology developed was packet-filter. A packet-filter firewall analyzes network traffic at the Transport layer of the OSI model. Each IP packet is examined to see if it matches a rule defining what data is allowed to pass through to the network.

The rules are configured by the network administrator. When you configure filtering, by default everything is blocked and you must designate what is allowed to pass through the firewall. The rules are based on information contained in the packet header.

* The source IP address
* The destination IP address
* The type of transport layer (TCP or UDP)
* The Transport layer's source por
* The Transport layers destination port
* The physical network interface though which the packet arrives
* The physical network interface though which the packet leaves

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