|  | 1.2 | LANs
| 1.2.4 | Logical (IP) addressing
An essential component of any network system is the process that enables information to locate specific computers systems on a network. Various addressing schemes are used for this purpose, depending on the protocol family being used. For example, AppleTalk addressing is different from TCP/IP addressing, which in turn is different from IPX addressing. Two important types of addresses are data link layer addresses and network layer addresses. Data link layer addresses, also called physical hardware addresses or MAC addresses , are typically unique for each network connection. In fact, for most LANs, data link layer addresses are located on the NIC (network interface card). Because a typical computer system has one physical network connection, it has only a single data link layer address. Routers and other systems that are connected to multiple physical networks can have multiple data link layer addresses. As their name implies, data link layer addresses exist at Layer 2 of the OSI reference model. Network layer addresses (also called logical addresses or IP addresses for the Internet Protocol suite) exist at Layer 3 of the OSI reference model. Unlike data link layer addresses, which usually exist within a flat address space, network layer addresses are usually hierarchical. In other words, they are like postal addresses that describe a person's location by indicating a country, state, ZIP Code, city, street, house address, and name. One example of a flat address is a U.S. Social Security number. Each person has a unique Social Security number, people can move around the country and obtain new logical addresses depending on their city, street, or ZIP Code, but their Social Security numbers remain unchanged.