next up previous

3.1 Internet and the Domain Name System     continued...

Figure 6: Local Network with Firewall.

Figure 6 shows a typical local area network and its relationship to a regional network. Every workstation and compute server supports TCP/IP and the applications built on top of this protocol. For example, users can log in to fog from mist using rlogin. Because there is a connection to the regional network (NorthWestNet) a user on fog can use rlogin to connect to compsci.

In many organizations the system that is connected to the regional network (e.g. drizzle in Figure 6) is the only one allowed to transfer packets outside the local network. The machine connected to the regional net is a firewall that isolates the other systems from the rest of the Internet. Given the recent history of Internet ``worms'' which, intentionally or not, invade local networks and waste resources, and the vulnerability of Unix systems to hackers who take it as a challenge to see how many systems they can log into, many system administrators prefer to connect only one system to the Internet and concentrate their efforts at maintaining security on this one system.