IP addresses (both IPv4 and IPv6) are, strictly speaking, assigned not to hosts or nodes, but to interfaces. In the most common case, where each node has a single LAN interface, this is a distinction without a difference. In a room full of workstations each with a single Ethernet interface
eth0 (or perhaps
Ethernet adapter Local Area Connection), we might as well view the IP address assigned to the interface as assigned to the workstation itself.
Each of those workstations, however, likely also has a loopback interface (at least conceptually), providing a way to deliver IP packets to other processes on the same machine. On many systems, the name “localhost” resolves to the IPv4 loopback address 127.0.0.1 (the IPv6 address ::1 is also used); see 7.3 Special Addresses. Delivering packets to the loopback interface is simply a form of interprocess communication; a functionally similar alternative is named pipes .
Loopback delivery avoids the need to use the LAN at all, or even the need to have a LAN. For simple client/server testing, it is often convenient to have both client and server on the same machine, in which case the loopback interface is a convenient (and fast) standin for a “real” network interface. On unix-based machines the loopback interface represents a genuine logical interface, commonly named
lo. On Windows systems the “interface” may not represent an actual operating-system entity, but this is of practical concern only to those interested in “sniffing” all loopback traffic; packets sent to the loopback address are still delivered as expected.
Workstations often have special other interfaces as well. Most recent versions of Microsoft Windows have a Teredo Tunneling pseudo-interface and an Automatic Tunneling pseudo-interface; these are both intended (when activated) to support IPv6 connectivity when the local ISP supports only IPv4. The Teredo protocol is documented in RFC 4380.
When VPN connections are created, as in 3.1 Virtual Private Networks, each end of the logical connection typically terminates at a virtual interface (one of these is labeled
tun0 in the diagram of 3.1 Virtual Private Networks). These virtual interfaces appear, to the systems involved, to be attached to a point-to-point link that leads to the other end.
When a computer hosts a virtual machine, there is almost always a virtual network to connect the host and virtual systems. The host will have a virtual interface to connect to the virtual network. The host may act as a NAT router for the virtual machine, “hiding” that virtual machine behind its own IP address, or it may act as an Ethernet switch, in which case the virtual machine will need an additional public IP address.
Routers always have at least two interfaces on two separate IP networks. Generally this means a separate IP address for each interface, though some point-to-point interfaces can be used without being assigned any IP address (7.12 Unnumbered Interfaces).
7.2.1 Multihomed hosts
A non-router host with multiple non-loopback network interfaces is often said to be multihomed. Many laptops, for example, have both an Ethernet interface and a Wi-Fi interface. Both of these can be used simultaneously, with different IP addresses assigned to each. On residential networks the two interfaces will often be on the same IP network (eg the same bridged Wi-Fi/Ethernet LAN); at more security-conscious sites the Ethernet and Wi-Fi interfaces are often on quite different IP networks (though see 7.9.5 ARP and multihomed hosts).
Multiple physical interfaces are not actually needed here; it is usually possible to assign multiple IP addresses to a single interface. Sometimes this is done to allow two IP networks (two distinct prefixes) to share a single physical LAN; in this case the interface would be assigned one IP address for each IP network. Other times a single interface is assigned multiple IP addresses on the same IP network; this is often done so that one physical machine can act as a server (eg a web server) for multiple distinct IP addresses corresponding to multiple distinct domain names.
Multihoming raises some issues with packets addressed to one interface, A, with IP address AIP, but which arrive via another interface, B, with IP address BIP. Strictly speaking, such arriving packets should be discarded unless the host is promoted to functioning as a router. In practice, however, the strict interpretation often causes problems; a typical user understanding is that the IP address AIP should work to reach the host even if the physical connection is to interface B. A related issue is whether the host receiving such a packet addressed to AIP on interface B is allowed to send its reply with source address AIP, even though the reply must be sent via interface B.
RFC 1122, §3.3.4, defines two alternatives here:
The Strong End-System model: IP addresses – incoming and outbound – must match the physical interface.
The Weak End-System model: A match is not required: interface B can accept packets addressed to AIP, and send packets with source address AIP.
Linux systems generally use the weak model by default. See also 7.9.5 ARP and multihomed hosts.
While it is important to be at least vaguely aware of the special cases that multihoming presents, we emphasize again that in most ordinary contexts each end-user workstation has one IP address that corresponds to a LAN connection.