In an environment that uses static IP addressing, when an administrator installs a new workstation, she looks up an available IP address and the corresponding subnet mask in the IP address table. This table may be written in a notebook or saved on a computer in a spreadsheet. Once she finds the IP address, she needs to determine the correct DNS (Domain Name System) and WINS (Windows Internet Naming Service) server addresses for the workstation to use. In addition, in a routed environment, the administrator needs to ascertain the correct default gateway address for the workstation's subnet. Then she manually configures the workstation with the proper TCP/IP information. For small networks or networks that do not experience many changes, this may be fine There are a number of downsides to administering IP addresses statically. Foremost is human error. If the administrator mistypes the IP address or subnet mask, the workstation may not have connectivity to the resources it requires. If the DNS or WINS server IP addresses are mistyped, the workstation will not be able to perform name resolution. If the default gateway is incorrect, the workstation will not be able to connect to remote subnets and resources. Another downside is maintaining the IP address table. The administrator must continually spend time viewing and searching the address table for available addresses. Once she finds an available address, the administrator must note in the table that the IP address is now in use. Finally, if the network is large and its users move about often, using static IP configurations can be frustrating and inefficient. Problems such as the ones described earlier are compounded with larger networks. == There are four methods of dynamically allocating IP addresses: automatic, dynamic, roaming, and manual. Three of these methods--dynamic, roaming, and manual--use DHCP to allocate the IP addresses. # Automatic allocation. Automatic IP addressing utilizes the client's operating system to allocate a private IP address. Microsoft's Windows 2000 and Windows 98, along with the Apple Mac OS 8.5 and later, are operating systems that support Automatic Private IP Addressing (APIPA). The theory behind APIPA is that small ad hoc networks will be able to achieve basic connectivity without the need for intervention by the administrator. APIPA allows a workstation to configure itself with an IP address in the absence of DHCP or any other IP addressing mechanism. Automatic allocation is a quick and easy solution to the IP addressing problem, but it is only useful in small networks that need basic connectivity without Internet access. Larger environments are typically subnetted to segment network traffic and increase performance. Since APIPA is limited to the 169.254/16 subnet, it cannot be utilized in those environments. The downside to using APIPA even in small networks is the difficulty it may cause in troubleshooting connectivity issues. # Dynamic allocation. Dynamic allocation uses DHCP as the mechanism to allocate IP addresses. The administrator assigns a range of addresses to the DHCP server. The DHCP server, in turn, assigns an IP address in the range to DHCP clients upon request. This range is known as a scope. The administrator, when defining a scope, also specifies the lease duration for any IP address assignments from the scope. A lease duration is the amount of time a DHCP client has exclusive use of an IP address. After the lease duration has expired, the DHCP client must request a new lease from a DHCP server. # Roaming allocation. Roaming allocation can be used in situations where there are areas that users may visit temporarily with their laptops. Such areas may be libraries, classrooms, laboratories, or conference rooms, where users will need a DHCP-allocated address for a brief period of time. The basic configuration of the roaming allocation method is much like the dynamic allocation method, with the notable exception that the lease duration time is very short for the scopes that service these areas. For example, a company may have a conference room where users want to utilize network resources via their laptops. For the roaming allocation method to work, the conference room LAN first needs to be segmented. This is required because a subnet can be serviced by only one scope at a time. The administrator then creates a scope for the conference room subnet. The scope is given a lease duration of about 45 minutes. When users connect to the conference room LAN, they receive an IP address from the conference room scope. Once they leave the conference room, the user can wait for the lease to expire, at which point the laptop will restart the DHCP conversation. They could also release the IP address and request a new one. The roaming allocation method is useful in small, local implementations. Although it can be used on a larger scale, the short lease duration may cause excessive DHCP traffic and additional load on the DHCP servers. # Manual allocation.Manual allocation is another method that can be used in situations where an administrator wants to know the MAC (Media Access Control) address of the DHCP client before assigning an IP address. An administrator may want to do this for security reasons, or she may simply want to know who is utilizing network resources for billing purposes. Manual allocation is typically used in academic settings. Manual allocation can also be used for network devices such as servers and network printers. In this case, the MAC address of the server is used to create a reservation. With reservations, changes can be made to the IP configurations of all servers in a particular scope or even the entire enterprise. For example, if an administrator wants all servers to point to another DNS server, she could simply change the Name Server option for the scope where the servers were located. When a server renews its address lease, it will receive the updated Name Server option. As you can see, manual allocation is a very time consuming and labor intensive. In essence, manual allocation is very similar to using BOOTP. It should be used only in environments that require knowledge of what devices are connecting to the network.

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