By default, TCP/IP considers any machine with multiple network interfaces to be a router. However, you can change a router into a multihomed host--a machine with more than one network interface that does not run routing protocols or forward IP packets. You typically configure the following types of machines as multihomed hosts:
NFS servers, particularly large data centers, can be attached to more than one network in order to share files among a large pool of users. These servers don't need to maintain routing tables.
Database servers can have multiple network interfaces for the same reason as NFS servers--to provide resources to a large pool of users.
Firewall gateways are machines that provide the connection between a company's network and public networks such as the Internet. Administrators set up firewalls as a security measure. When configured as a firewall, the host will not pass packets between the networks attached to it. On the other hand, it can still provide standard TCP/IP services, such as ftp or rlogin, to authorized users.
Since TCP/IP considers any machine with multiple network interfaces to be a router, you need to perform a few operations to turn it into a multihomed host.