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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.