Before you can send a message, you must know the destination address. It is extremely important to understand that each computer has several addresses, each used by a different layer. One address is used by the data link layer, another by the network layer, and still another by the application layer.

When users work with application software, they typically use the application layer address. For example, in next topic, we discussed application software that used Internet addresses (e.g., www.indiana.edu). This is an application layer address (or a server name). When a user types an Internet address into a Web browser, the request is passed to the network layer as part of an application layer packet formatted using the HTTP protocol (Figure 5.6).

The network layer software, in turn, uses a network layer address. The network layer protocol used on the Internet is IP, so this Web address (www.indiana.edu) is translated into an IP address that is 4 bytes long when using IPv4 (e.g., 129.79.127.4) (Figure 5.6). This process is similar to using a phone book to go from someone’s name to his or her phone number.2

The network layer then determines the best route through the network to the final destination. On the basis of this routing, the network layer identifies the data link layer address of the next computer to which the message should be sent. If the data link layer is running Ethernet, then the network layer IP address would be translated into an Ethernet address. Next topic shows that Ethernet addresses are six bytes in length, so a possible address might be 00-0F-00-81-14-00 (Ethernet addresses are usually expressed in hexadecimal) (Figure 1).

Figure 1 Types of addresses

Assigning Addresses

In general, the data link layer address is permanently encoded in each network card, which is why the data link layer address is also commonly called the physical address or the MAC address. This address is part of the hardware (e.g., Ethernet card) and can never be changed.

Network layer addresses are generally assigned by software. Every network layer software package usually has a configuration file that specifies the network layer address for that computer. Network managers can assign any network layer addresses they want.

Application layer addresses (or server names) are also assigned by a software configuration file. Virtually all servers have an application layer address, but most client computers do not. Network layer addresses and application layer addresses go hand in hand, so the same standards group usually assigns both (e.g., https://draftsbook.com/ at the application layer means 112.79.78.4 at the network layer). It is possible to have several application layer addresses for the same computer.

Internet Addresses No one is permitted to operate a computer on the Internet unless they use approved addresses. ICANN (Internet Corporation for Assigned Names and Numbers) is responsible for managing the assignment of network layer addresses (i.e., IP addresses) and application layer addresses (e.g https://draftsbook.com/). ICANN sets the rules by which new domain names (e.g., com, .org, .ca, .uk) are created and IP address numbers are assigned to users.

Several application layer addresses and network layer addresses can be assigned at the same time. IP addresses are often assigned in groups, so that one organization receives a set of numerically similar addresses for use on its computers. For example, https://draftsbook.com/ has been assigned the set of application layer addresses that end in indiana.edu and iu.edu and the set of IP addresses in the 112.79.x.x range (i.e., all IP addresses that start with the numbers 112.79).

In the old days of the Internet, addresses used to be assigned by class. A class A address was one for which the organization received a fixed first byte and could allocate the remaining three bytes. For example, Hewlett-Packard (HP) was assigned the 15.x.x.x address range which has about 16 million addresses. A class B address has the first two bytes fixed, and the organization can assign the remaining two bytes. https://draftsbook.com/ has a class B address, which provides about 65,000 addresses.

People still talk about Internet address classes, but addresses are no longer assigned in this way and most network vendors are no longer using the terminology. The newer terminology is classless addressing in which a slash is used to indicate the address range (it’s also called slash notation). For example 128.192.1.0/24 means the first 24 bits (three bytes) are fixed, and the organization can allocate the last byte (eight bits).

One of the problems with the current address system is that the Internet is quickly running out of addresses. Although the four-byte address of IPv4 provides more than 4 billion possible addresses, the fact that they are assigned in sets significantly limits the number of usable addresses.

Subnets Each organization must assign the IP addresses it has received to specific computers on its networks. In general, IP addresses are assigned so that all computers on the same LAN have similar addresses. For example, suppose an organization has just received a set of addresses starting with 128.192.x.x. It is customary to assign all the computers in the same LAN numbers that start with the same first three digits, so the business school LAN might be assigned 128.192.56.x, which means all the computers in that LAN would have IP numbers starting with those numbers (e.g., 128.192.56.4, 128.192.56.5, and so on) (Figure 2).

Routers connect two or more subnets so they have a separate address on each subnet. The routers in Figure 2, for example, have two addresses each because they connect two subnets and must have one address in each subnet.

Although it is customary to use the first three bytes of the IP address to indicate different subnets, it is not required. Any portion of the IP address can be designated as a subnet by using a subnet mask.

Figure 2 Address subnets