The IT Law Wiki


An IP address (also called an IP number or Internet Protocol address) is

[a] 32-bit binary number that uniquely identifies a host connected to the Internet or to other Internet hosts for communication through the transfer of data packets. An IP address is expressed in “dotted quad” format consisting of decimal values of its four bytes separated with periods, e.g.,[1]

An IP address is "[t]he unique identification of the location of an end-user's computer."[2]


It is used to identify any device, such as a computer or a printer connected to the Internet.[3] An IP address is a string of four sets of numbers (4 bytes), written as a decimal number, separated by periods. For example, a typical IP address might be[4]

In decimal notation, a byte (which is made up of 8 bits) can have a value between 0 and 255. Thus a valid IP addresses can use only the numbers between 0 and 255 in the dotted-decimal notation. The numbers 0 and 255 in the network or host address part of the IP address have special meanings.

The IP address does not denote a physical location of the device at the time it is connected to the Internet.


IP Address Assignments[]

Every device connected to the Internet (or group of devices using the same account to access the Internet) must be assigned an IP address so that Internet traffic sent from and directed to thatdevice is directed properly from its source to its destination.

ISP’s assign IP addresses to their customers’ computers. An ISP might assign a different IP address to a customer each time the customer makes an Internet connection ("dynamic IP address"), or it might assign an IP address to a customer permanently or for a fixed period of time ("static IP address"). Either way, the IP address used by a computer attached to the Internet must be unique for the duration of a particular session; that is, from connection to disconnection. ISP’s typically log their customers’ connections, which means that the ISP can identify which of its customers was assigned a specific IP address during a particular Internet session.

When a message or data packet is sent over the Internet, the routing systems ensure that the packet is delivered to appropriate devices, by consulting the destination IP address placed into the packet by the sender. To avoid every router having to know the location of every machine, the address space is arranged in a hierarchical manner with blocks of IP addresses (of varying sizes from hundreds to millions) being allocated to Internet Service Providers (ISPs). The ISPs then make allocations from these blocks to their individual customers. Thus routers need only ascertain the address block and relay the packet to the appropriate ISP. Once the packet arrives at the ISP, it can use more fine-grained routing information to deliver it to the correct device.

Traceability of IP Addresses[]

When a new connection is made to a computer that is offering an Internet service, it will determine where to respond by inspecting the "source address" of the incoming packet. It then sends a packet back to that source, and — provided that an acceptable reply is received from that source (some random numbers are included in these "handshake packets" to prevent spoofing) — it will then open the connection and be prepared to send and/or receive real data.

If the connection turns out to be abusive — for example, it is an incoming spam email advertising fake medicines — then the source address can be traced back by determining which block of addresses it comes from, and hence which ISP allocated the address. The records at that ISP can then identify the customer to whom the IP address was issued. Since many ISPs allocate the same address to different customers at different times, the exact time of the connection will often be needed, in order to correctly identify the customer who was using this dynamic IP address.

This "traceability" of IP addresses permits the identification of the source ISP — who may be prepared to act to prevent further abuse. It also permits the identification of the customer account, although the ISP may not be prepared to divulge this information until the necessary legal paperwork has been processed in the appropriate jurisdiction.


  1. U.S. Department of Justice, National Institute of Justice, Electronic Crime Scene Investigation: A Guide for First Responders 55 (2d ed. Apr. 2008) (full-text).
  2., Inc. v. Verio, Inc., 356 F.3d 393, 407 n.4 (2d Cir. 2004).
  3. IP addresses are considered personal data in some countries.
  4. IP addressing uses four decimal-separated numbers, which allows for a total of 256^4 or 1,099,511,627,776 unique addresses. This addressing scheme is being expanded to accommodate additional Internet usage. The “IPv6”, a more recent version of the Internet Protocol, uses 128-bit addresses. Regardless of the addressing scheme used, the method of tracing the IP address will likely remain the same.