The only way that anyone outside the intranet can get to the intranet is through a router, so it makes sense that the router will be the first place to put security rules into place. Routers also can be used within intranets to prevent internal security breaches.

Routers examine all packet coming into and going out of an intranet and analyse where to send those packets so that they can be delivered to the correct or proper address. They can control the type and direction of traffic permitted and essentially can also decide whether packets should even be delivered. In other words, they can block certain packets from coming into or going out of an intranet.

When routers are used in this way-to protect an intranet by blocking sure packets-they are called filtering routers or screening routers.

An  administrator establishes a filtering table that contains a lot of  rules about which packets are allowed to pass and which packets to be dropped. Every packet coming into and going out of an intranet has a value of layers of information in it. These layers contain the data being sent and information about the kind of Internet resource being used (FTP, Telnet, and so forth), the source address and destination address of the packet, and other information. Filtering routers use the information in those layers to evaluate which rules in the filtering table apply to each packet. When packets pass through the router, the router examines the packets, looks at the filtering table, and then decides which action to take. The * wild card can be used at the end of IP addresses, for example, to apply rules to entire subnets or servers.
A filtering router can allow people from inside an intranet to use Telnet, but not allow anyone outside the intranet to Telnet into the intranet. It can block specific source addresses from accessing the intranet. A filtering router distinguishes between input and output ports traffic.
Different rules can be set up for incoming packets and outgoing packets so that intranet users can be given access to Internet services, while anyone on the Internet could be banned from accessing certain intranet services and data.
Filtering routers can keep logs about filtering activity. Commonly, they track packets not allowed to pass between the Internet and the intranet, which would indicate an intranet has been under attack.

The router examines the data in the IP header which wraps the data and the transport layer header information. That means that any given packet will have data in it, as well as two sets of headers-one from the transport layer, and one from the Internet layer. Filtering routers examine all these data and headers to decide whether to let packets pass.
Source addresses are read from the IP header and compared to the source address listings in the filtering tables. Certain addresses may be known to be dangerous and including them in the table allows the router to drop that traffic.

Routers can have different rules for subnets since they may require different levels of security. A subnet that contained highly private financial or competitive information might have many restrictions. An engineering subnet may have few restrictions on incoming or outgoing activities.
A filtering router can allow users to have access to services like Telnet and FTP, while restricting Internet use of these services to access the intranet. This same technique can be used to prevent internal users from accessing restricted data on an intranet. For example, it can allow finance members outgoing use of FTP while dropping FTP requests from the engineering department into the finance department.

Certain kinds of services are more dangerous than others. For example, FTP is used to download files but may bring files containing a virus. Telnet and the rlogin command (like Telnet but with a greater risk for security break-ins) are banned by rules in the filtering table that evaluate this type of service by the source or destination port number. Telnet addresses port 23, and rlogin port 513.

Address spoofing is a common method of attack. In address spoofing, someone from outside the intranet forges a source address so that it looks to a router as if the source address is really someone from inside the intranet. The spoofer hopes to trick the filtering router into allowing greater access to the intranet than would be allowed an external originating address. Once the router was convinced that the spoofer was already inside the intranet, private files potentially could be sent outside the intranet.

Filtering routers have a way of handling address spoofing. A rule can be established that tells the router to look at the source address in every incoming-but not outgoing-IP header. If the source address is internal, but the packet is coming from outside the intranet, the router would drop the packet.