A firewall is a security system, designed to prevent unauthorized access to a private or local network. The system can either be a hardware firewall or a software firewall, or a combination of both.

The general use of firewalls on the Internet is to prevent Internet users from accessing local networks or Intranets.

A firewall is used to prevent unauthorized access to a network. A network can be made up of a company's local area networks, wide area networks, and the Internet, or it can be just an Internet Service Provider preventing inappropriate access to the files of its customers.

The firewall's role is to ensure that all communication to and from the organisation and the Internet conforms to the organization's security policies.

In general, firewalls permit one-way access to the Internet. Because RealServer and RealPlayer need to establish two-way communication to stream and receive media content, firewalls may reject RealPlayer's attempt to establish this connection, and the RealPlayer's request for a clip will "bounce" off the firewall.

RealNetworks designed both RealPlayer and RealServer to work with a firewall while still protecting the company's internal networks. By making a few quick changes to your firewall, RealServer, RealPlayer, or a combination of the three, you can still use the security advantages of a firewall while enjoying streaming media.

Firewalls generally fall into two categories: application-level firewalls (such as proxies) and network-level firewalls (such as packet filters).

Many organizations use a combination of these two types of firewalls to achieve a higher level of security than either alone can provide.

Application-level firewalls first determine if a requested connection between a computer on the internal network and one on the outside is permitted. If the connection is authorized the firewall sets up the necessary communication links between the two computers. As an intermediary, the firewall can monitor the communication between the two networks and suppress any unauthorized activity.

Rather than impersonating an application, as do application-level firewalls, network-level firewalls examine the packets of information sent at the transport level to detetermine whether a particular packet should be blocked. Each packet is either forwarded or blocked based on a set of rules defined by the firewall administrator.

A common configuration for network-level-filtering firewalls is to allow all connections initiated by machines inside the firewall, and restrict all connections for machines outside of the firewall. For most programs, this works well since they usually only establish a single outbound TCP connection.