We wouldn't have file-sharing lawsuits at all without the ability to convert a computer's IP address into an Internet subscriber's real name. But how do Internet providers actually perform those lookups?

The process begins when a copyright holder files suit and supplies the court with a list of IP addresses that were allegedly seen sharing files online. The court then allows the rightsholder to send a subpoena to the Internet service provider (ISPs) responsible for each address, where it goes to a special compliance unit.

These units are not large. Time Warner Cable told a DC federal court last year that its own subpoena compliance unit had only four full-time employees (but it had been forced to add a temp worker due to the growing number of requests). Comcast told a New York federal court recently that it processes around 200 lookups a day and that each employee can handle 22 of these, putting the department at nine employees.

Most of these lookups are done for law enforcement, and many of them are emergency requests. "Such requests often involve an imminent threat of death or injury or the imminent flight of a suspect," says Comcast, "and often require compliance within less than an hour after receiving the request."

Time Warner receives an average of 567 IP lookup requests a month, almost all of which came from law enforcement before the recent wave of P2P filesharing lawsuits. The requests include information needed to investigate "suicide threats, child abduction cases, and terrorist activity," which get "immediate priority," while others are a bit less pressing.

Both companies agree that suing individual file-swappers in court has placed huge burdens on their compliance operations—operations that make them no money. For instance, when 2,000 people were sued last year for sharing the film Far Cry online, Time Warner was asked to look up addresses for 809 of them—and that was just in one case, which was later expanded to over 4,000 people. With many other cases being filed in 2010, nearly 100,000 US citizens were sued, with ISPs being asked to do lookups on every one of them.

According to Time Warner, the burden is almost unmanageable. Just a handful of these P2P cases would "take TWC nearly three months of full time work by TWC's Subpoeana Compliance group," the company told the court, "and TWC would not be able to respond to any other requests, emergency or otherwise, from law enforcement during this period."

TWC asked the court to limit such lookups from one law firm to 28 per month, even though it would take years to clear the backlog—and the court eventually agreed. Comcast has tried to limit requests from a single law firm to 25 per month, despite its higher staffing levels (the company is trying to double its lookup capacity in order "to handle the onslaught of copyright infringement orders it receives weekly that were not forecasted for its 2010 budget.")

"Multiple people in multiple locations"

The actual lookup is not a purely automated process. Here's how Comcast does it:

Comcast is able to identify which subscriber account was assigned a particular IP address on a specific date at a specific time. However, because IP addresses are dynamically assigned and users can change day-to-day (and within a day), Comcast has to consult its logs for each date and time associated with each IP address to determine the identity of the subscriber account assigned to that IP address. The logs identify a specific numerical address for a cable modem and then Comcast has to consult its subscriber database(s) to see which subscriber has that modem registered to his or her account. Whether an IP address links to a subscriber account already identified in connection with another look-up, or does not link at all, the look-up process for identifying an IP address is the same whether or not it produces unique identifying information. In addition, if the requests concern IP address usage more than 180 days prior to the request, Comcast cannot identify a dynamic IP subscriber as logs are only maintained for a period of 180 days; however, Comcast must go through the entire process to look up the IP address because business class static IP addresses are available more than 180 days back and there is no identification on the Subpoena as to what type of subscriber the IP belongs to. Hence the same amount of work is put forth in the initial identification process.

Just to add to the tedium, each lookup request is done by one analyst but then vetted by a second analyst for accuracy.

And lookups aren't returned to the requesting attorneys at all until a subscriber is first notified and given a brief chance to object; Comcast sends out such notices by overnight mail.

Time Warner's process is similar, though the company notes that it requires action at "the corporate level and the local level," and it requires the cooperation of "multiple people at multiple locations."

Such lookups are therefore expensive to do. Each lookup costs Comcast about $120, which "includes the look-ups through two systems, Comcast's labor to confirm authenticity and accuracy, notification by overnight mail, quality control review, and interaction with responding subscribers and publication to the plaintiff via facsimile or CD." Time Warner says its costs are $45 per lookup, though this does not appear to include subscriber notification.

It's a tough process to do a simple job—tell the lawyers which subscriber had a particular IP address at a particular time.