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DHCP for Windows 2000 by Neall Alcott

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Creating an IP Addressing Plan

Before deciding to implement DHCP, an administrator must first decide on an IP addressing plan. There are many different ways to create an IP addressing plan, and in some cases they may need to be combined. This is a critical step because it is the foundation of the entire DHCP infrastructure. This section looks into each of the different methods, describing their benefits, how they may be implemented, and some of their limitations.

Static IP Addressing

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 and WINS 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.

One of the downsides to this method of administering IP addresses 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. Also, by storing the address table in a notebook, the table could easily be lost. Even storing the address table in a spreadsheet does not lessen the chance that it will become corrupted or lost.

Moreover, 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. Incorrect configurations have a much larger effect on connectivity, as the workstation routinely needs access to resources on different subnets. Maintaining the IP address table centrally becomes nearly impossible. In all likelihood, the address table would need to be divided along subnets and individually maintained by the local administrator.

Static IP addressing can also be a huge liability if the organization needs to redesign their entire addressing structure. Factors that cause organizations to change their addressing structure include mergers and acquisitions, changing Internet service providers (ISPs), or network growth. Changing IP address configurations enterprise-wide requires an administrator to visit each workstation, server, and network device. In the end, it costs the organization a lot of time and money.

In short, as the network’s capacity and scope grows in size, static IP address administration becomes unwieldy and inefficient.

Dynamic IP Addressing

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 MacOS 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.

An example of an ad hoc network would be a dentist’s office. The dentist has 5 separate computers. One night at a dinner party, a friend tells him of all the benefits she is reaping from her new computer network. The dentist decides that he too could benefit from a network. He buys the necessary cabling and hooks everything together. Typically at this point things start getting difficult. It is likely that he doesn’t have a deep understanding of the Windows 2000 operating system or the TCP/IP protocol. However, with APIPA, the computers will be automatically configured. In the end, the dentist will have a functioning network.

APIPA allows a workstation to configure itself with an IP address in the absence of DHCP or any other IP addressing mechanism. Other networking protocols, such as IPX/SPX and Appletalk, already include this type of functionality.

Creating small ad hoc networks can be very useful in environments such as small businesses and homes that include only a few machines. In order for the machines to communicate, they must be configured with IP addresses.

Computers using APIPA must also be DHCP clients. This is because APIPA uses the DHCP client to determine whether a DHCP server is available.

Using the DHCP client, the computer requests an IP address by sending a DHCPDISCOVER message. After not receiving a response, the computer automatically configures itself with an IP address in the reserved Class B network 169.254.0.0 and a subnet mask of 255.255.0.0. The DHCP client then performs a duplicate address check by sending an ARP request for the IP address. If it receives a response, it determines that the address is in use. At this point it selects another address from the 169.254.0.0 subnet and again performs a duplicate address check. The client repeats this process for up to 10 addresses, after which the automatic addressing fails.

Automatic allocation is a quick and easy solution to the IP addressing problem, but 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. For example, if an administrator has workstations on a network and wants to assign these workstations addresses in the 192.168.1.0/24 subnet, he creates a DHCP scope that consists of the IP addresses 192.168.1.1 through 192.168.1.254. When a DHCP client requests an address from the DHCP server, the server assigns one of these addresses.

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 that a DHCP client has exclusive use of an IP address. With DHCP, the client has two opportunities to extend the lease, first when the lease duration is 50% complete and then again when the lease duration is 87.5% complete. After the lease duration has expired, the DHCP client must request a new lease from a DHCP server.

The administrator, if needed, can also exempt certain addresses from the scope. These addresses may be network devices or hosts whose IP addresses should not change, for example, network printers, routers, and servers. The administrator can set aside a portion of the scope, say 192.168.1.1 through 192.168.1.25, for these devices. Now when a DHCP client requests an IP address, the DHCP server assigns an address between 192.168.1.26 through 192.168.1.254. Another option for network devices such as these would be to configure a DHCP reservation, where the DHCP server allocates the same IP address to the device’s MAC address.

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.

If the RRAS server contains more than one LAN interface, by default it will choose the LAN interface randomly. You can override this action by unselecting the “Allow RAS to select adapter” checkbox under IP properties and selecting the desired interface. This setting can be found in the Routing and Remote Access Microsoft Management Console (MMC).

Manual allocation

Manual allocation is another method that can be used in situations where an administrator wants to know the MAC address of the DHCP client before assigning an IP address. An administrator may want to do this for security reasons, or may simply want to know who is utilizing network resources for billing purposes. Manual allocation is typically used in academic settings.

The manual allocation process begins when a user wants to install a new computer or device on the network. The user must submit a request to the administrator that includes his computer’s MAC address and its physical location (i.e., building and room number). Once the administrator receives the request, she configures an IP address reservation on the DHCP server. This reservation is placed in the appropriate subnet scope (i.e., the user’s physical location) using the user’s MAC address (i.e., the user’s computer). Once notified that everything has been set up, the user can then boot the workstation. The workstation then obtains the IP address from the DHCP server.

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 very time consuming and labor intensive. In essence, manual allocation is very similar to using BOOTP. It should only be used in environments that require knowledge of what devices are connecting to the network.

Combining dynamic addressing methods

Some of these methods can be combined and intertwined to create the DHCP solution an organization requires. The only one that cannot be combined is the automatic allocation method (unless your network is going to use the 169.254.0.0 subnet, of course).

For example, an organization may want to use static IP addressing for some network devices, such as network printers and file servers, while using the dynamic allocation method for the rest of their network. They can also create some subnets for conference rooms and libraries using the roaming allocation method.

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