“The” IP Address

As elaborated in a prior post, sites and services often try to use an IP address as an input into protection (e.g. blocking a spammer) or customization (providing local weather info) algorithms. Commonly, software would like to know “What is this device’s IP address?” Unfortunately, this is not a trivial question to answer, because it makes several incorrect assumptions: That the client device only has one IP address That network packets received by a service from the client device will have that IP address as the remote_addr. Both of these assumptions are incorrect, in several ways. Not One Address Devices may have multiple network adapters, each with their own address. Those adapters may be “physical” (e.g. a network card) or “virtual” (e.g. a VPN adapter). Each network adapter may itself have multiple addresses, which is almost universally true for modern devices that support both IPv4 and IPv6. Client Perspective != Remote Perspective It is commonly the case that client requests…