This section discusses methods of assigning IP addresses to end systems and explains their influence on administrative overhead. Address assignment includes assigning an IP address, a default gateway, one or more domain name servers that resolve names to IP addresses, time servers, and so forth. Before selecting the desired IP address assignment method, the following questions should be answered:
■ How many devices need an IP address?
■ Which devices require static IP address assignment?
■ Is IP address renumbering expected in the future?
■ Is the administrator required to track devices and their IP addresses?
■ Do additional parameters (default gateway, name server, and so forth) have to be configured?
■ Are there any availability issues?
■ Are there any security issues?
Static Versus Dynamic IP Address Assignment Methods
Following are the two basic IP address assignment strategies:
■ Static: An IP address is statically assigned to a system. The network administrator configures the IP address, default gateway, and name servers manually by entering them into a special file or files on the end system with either a graphical or text interface. Static address assignment is an extra burden for the administrator—especially on large-scale networks— who must configure the address on every end system in the network.
■ Dynamic: IP addresses are dynamically assigned to the end systems. Dynamic address assignment relieves the administrator of manually assigning an address to every network device. Instead, the administrator must set up a server to assign the addresses. On that server, the administrator defines the address pools and additional parameters that should be sent to the host (default gateway, name servers, time servers, and so forth). On the host, the administrator enables the host to acquire the address dynamically; this is often the default. When IP address reconfiguration is needed, the administrator reconfigures the server, which then performs the host-renumbering task. Examples of available address assignment protocols include Reverse Address Resolution Protocol, Boot Protocol, and DHCP. DHCP is the newest and provides the most features.
When to Use Static or Dynamic Address Assignment
To select either a static or dynamic end system IP address assignment method or a combination of the two, consider the following:
■ Node type: Network devices such as routers and switches typically have static addresses. End-user devices such as PCs typically have dynamic addresses.
■ The number of end systems: If there are more than 30 end systems, dynamic address assignment is preferred. Static assignment can be used for smaller networks.
■ Renumbering: If renumbering is likely to happen and there are many end systems, dynamic address assignment is the best choice. With DHCP, only DHCP server reconfiguration is needed; with static assignment, all hosts must be reconfigured.
■ Address tracking: If the network policy requires address tracking, the static address assignment method might be easier to implement than the dynamic address assignment method. However, address tracking is also possible with dynamic address assignment with additional DHCP server configuration.
■ Additional parameters: DHCP is the easiest solution when additional parameters must be configured. The parameters have to be entered only on the DHCP server, which then sends the address and those parameters to the clients.
■ High availability: Statically assigned IP addresses are always available. Dynamically assigned IP addresses must be acquired from the server; if the server fails, the addresses cannot be acquired. To ensure reliability, a redundant DHCP server is required.
■ Security: With dynamic IP address assignment, anyone who connects to the network can acquire a valid IP address, in most cases. This might be a security risk. Static IP address assignment poses only a minor security risk.
The use of one address assignment method does not exclude the use of another in a different part of the network.
Guidelines for Assigning IP Addresses in the Enterprise Network
The typical enterprise network uses both static and dynamic address assignment methods. As shown in Figure 6-14, the static IP address assignment method is typically used for campus network infrastructure devices, in the Server Farm and Enterprise Data Center modules, and in the modules of the Enterprise Edge (the E-Commerce, Internet Connectivity, Remote Access and VPN, and WAN and MAN and Site-to-Site VPN modules). Static addresses are required for systems such as servers or network devices, in which the IP address must be known at all times for connectivity, general access, or management.
Figure 6-14 IP Address Assignment in an Enterprise Network
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