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Firewalls prevent unauthorized access to or from a private network, such as
intranets or any kind of network including the home networks. They are
basically filters and can be either hardware or software or combination of
both. All messages to and from the network are analyzed and some are discarded
if they don't meet the criteria of this network.
A firewall is normally installed in a seperate computer from the rest of the
network so that no incoming IP packet can get directly at private network
resources before checked.
They use a few methods to control traffic to and from the network.
Packet Filtering : This method checks every packet against the
predefined set of rules and the ones that does not meet the criteria are
discarded and the rest is passed through.
Proxy service : Intercepts all messages entering and leaving the network.
By this you can also hide the true network address.
Circuit level check : Checks are applied at the TCP or UDP connection is
established. Once this is done, no further checks are needed.
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