IP address By using the IP address as a parameter, the firewall can allow or deny traffic, based on the source or destination IP address. For example, you can configure the firewall so that only certain hosts on the internal network are able to access hosts on the Internet. Alternatively, you can configure it so that only certain hosts on the Internet are able to gain access to a system on the internal network.
Port number, the TCP/IP suite uses port numbers to identify which service a certain packet is destined for. By configuring the firewall to allow certain types of traffic, you can control the flow. You might, for example, open port
80on the firewall to allow Hypertext Transfer Protocol (HTTP) requests from users on the Internet to reach the corporate Web server. You might also, depending on the application, open the HTTP Secure (HTTPS) port, port
443, to allow access to a secure Web server application.
Protocol ID Because each packet transmitted with IP has a protocol identifier in it, a firewall can read this value and then determine what kind of packet it is. If you are filtering based on protocol ID, you specify which protocols you will and will not allow to pass through the firewall.
MAC address This is perhaps the least used of the packet-filtering methods discussed, but it is possible to configure a firewall to use the hardware-configured MAC address as the determining factor in whether access to the network is granted. This is not a particularly flexible method, and it is therefore suitable only in environments in which you can closely control who uses which MAC address. The Internet is not such an environment.
Of the firewall methods discussed in this chapter, packet filtering is the most commonly implemented. Packet filtering enables the firewall to examine each packet that passes through it and determine what to do with it, based on the configuration. A packet-filtering firewall deals with packets at the data-link and network layers of the Open Systems Interconnect (OSI) model. The following are some of the criteria by which packet filtering can be implemented: