If you use TCP/IP on a large intranet with high volume or multiple subnets, you probably use a third-party router and a leased-line connection to the Internet. If your third-party routers create a firewall by filtering packets, you can use the router as an Internet gateway.
You can specify exactly which packets are routed from the intranet to the Internet and vice versa. Typically, you do not permit any unrequested packets to enter your intranet and you specify the intranet users who are permitted access to the Internet. Router configuration and topology for this type of application is often provided by the router vendor.
Figure 3.10 Third-party TCP/IP router security
For more information about third-party router capabilities, see your third-party vendor or third-party router documentation. For more information about using routing with Windows NT Server, see Chapter 4 in the Windows NT Server Networking Supplement.