Routing is the act of moving information across an internetwork from a source to destination. Along the way, at least one intermediate node typically is encountered. Routing is often contrasted with bridging, which might seem to accomplish precisely the same thing to the casual observer. This distinction provides routing and bridging with different information to use in the process of moving information from source to destination, so the two functions accomplish their tasks in different ways.

Routing involves two basic activities: determining optimal routing paths and transporting information groups (typically called packets) through an internetwork. In the context of the routing process, the latter of these is referred to as packet switching.

Although packet switching is relatively straightforward, path determination can be very complex. Switching algorithms is relatively simple; it is the same for most routing protocols. In most cases, a host determines that it must send a packet to another host.

Having acquired a router’s address by some means, the source host sends a packet addressed specifically to a router’s physical (MediaAccess Control (MAC)-layer) address, this time with the protocol (network layer) address of the destination host. As it examines the packet’s destination protocol address, the router determines that it either knows or does not know how to forward the packet to the next hop. If the router does not know how to forward the packet, it typically drops the packet.

Setting up a firewall seems to be easy and straightforward for most users. It’s nothing more than installing a piece of software, then allowing or blocking network traffic caused by applications running on the user’s computer by means of a few mouse clicks. The user sits behind the nice little brick wall running on his or her computer that assures of safety. Before being able to understand a discussion of firewalls, it’s important to understand the basic principles that make firewalls work.

A firewall is a system or group of systems that enforces an access control policy between two or more networks. The actual means by which this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms: one which exists to block traffic, and the other which exists to permit traffic. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic.

Probably the most important thing to recognize about a firewall is that it implements an access control policy. If you don’t have a good idea of what kind of access you want to allow or to deny, a firewall may not help you.It’s also important to recognize that the firewall’s configuration, because it is a mechanism for enforcing policy.

A firewall’s purpose is to keep those out of your network, who can harm your users, applications, systems and business, while protecting important information and proprietary data and allowing you to do your job.