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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Addressing Concepts and Issues

IP Addressing Categories (Classful, Subnetted and Classless) and IP Address Adjuncts (Subnet Mask and Default Gateway) 1 2 IP Address Management and Assignment Methods and Authorities

Using a Multihomed Host as a Router

Now, let's consider the second case. If a host has interfaces to two or more different networks, then could it not pass IP datagrams between them? Of course, it certainly could, if it had the right software running on it. If it does this, wouldn't that make the host a router, of sorts? In fact, that is exactly the case! A multihomed host with interfaces to two networks can use software to function as a router. This is sometimes called software routing.

Using a host as a router has certain advantages and disadvantages compared to a hardware router. A server that is multihomed can perform routing functions and also, well, act as a server. A dedicated hardware router is designed for the job of routing and usually will be more efficient than a software program running on a host.

Multihoming was once considered a fairly “esoteric” application, but has become more common in recent years. This is also true of multihoming on different networks for software routing use. In fact, you may be doing this in your home without realizing it!

Suppose you have two PCs networked together and a single phone line to connect to the Internet. One computer dials up to your Internet Service Provider, and runs software such as Microsoft's Internet Connection Sharing (ICS) to let the other computer access the Internet. Millions of people do this every day—they have a multihomed system (the one connecting to the Internet and the other PC) with ICS acting in the role of a software router (though there are some technical differences between ICS and a true router, of course.)

IP Addressing Categories (Classful, Subnetted and Classless) and IP Address Adjuncts (Subnet Mask and Default Gateway) 1 2 IP Address Management and Assignment Methods and Authorities