Appendix A: IP Addressing
In any network, every destination must have a unique identiﬁer that other machines can use to send information. This unique identiﬁer is commonly referred to as an address. An IP (Internet Protocol) address is a unique identiﬁer for a node or host connection on an IP network. In reality, on an IP network, an address identiﬁes a network attachment point as opposed to a particular machine. Hence, single machines with multiple interfaces (i.e., network interface cards) will have multiple IP addresses — one for each interface. Machines with multiple addresses are called multi-homed machines because they live on more than one network. A router is a multi-homed device, but not all multi-homed devices are routers. You might have ﬁle servers and ﬁrewalls residing on more than one network.