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How to determine when to use reserved IP addresses
Network admins choose IP address configuration based on management requirements. Each address type has a specific role, but reserved IP addresses are best in certain situations.
Network professionals recognize the importance of correctly configuring IP addresses on network nodes. IP addresses -- and their related subnet masks -- are critical to network communications.
The two standard ways to configure IP addresses are the following:
- Static. An administrator manually configures an IP address configuration.
- Dynamic. The node leases an IP address configuration from a Dynamic Host Configuration Protocol (DHCP) server.
Most network environments use both static and dynamic IP addresses. Devices such as servers and routers receive static configurations. End-user workstations rely on dynamic DHCP settings.
Static IP address management is more time-intensive and prone to typographical errors, but it ensures the node has an unchanging IP address on the network. Another way to do this is through DHCP reservation.
Network administrators reserve IP addresses for specific clients on DHCP servers. When those clients initiate the IP address lease generation process, the DHCP server identifies them and offers the same IP settings every time. This is helpful for systems that need specialized or unique configurations, and it avoids the difficulties of manual configuration.
Using reserved IP addresses presents a viable alternative in certain situations. The choice between the two comes down to the different use cases for each option.
Static IP address use cases
Many devices require fixed IP addresses, which means the network must preserve their network identities through reboots or downtime. This is necessary in situations such as when servers host directories shared over a network. Network administrators often configure client systems to recognize these servers by IP address, and this requires the addresses to remain consistent over time. Routers, enterprise switches and print devices are also candidates for static IP address settings.
Consistent configuration is a benefit but comes at a price. Manual configurations are difficult to maintain and document. Administrators must track static IP address settings carefully to avoid duplicate identities on the network and prevent typographical errors. In addition to entering the IP address by hand, network administrators must also include the subnet mask and the default gateway address.
Despite the benefits, it's often challenging and time-consuming to reconfigure multiple systems with static IP address settings.
Network administrators configure static IP address settings in the properties of the network interface card using the interface of the host OS. The interface varies slightly among Linux, macOS and Windows systems.
Reserved IP address use cases
Dynamic IP addressing begins with a DHCP server configured with a scope of IP addresses available to client devices. As the server comes online, each client system goes through the DHCP lease generation process to request the use of an address and its related settings. This is more flexible and easier to update than static IP settings.
The DHCP lease generation process is as follows:
- Discover. The client broadcasts a DHCP Discover message on the network. The message includes the client's MAC address.
- Offer. The DHCP server broadcasts a DHCP Offer message to offer an IP configuration. The message is addressed to the client's MAC address.
- Request. The client broadcasts a DHCP Request message to ask to use the offered IP settings.
- Acknowledge. The DHCP server sends a DHCP Acknowledge message, which confirms the IP address configuration lease.
When an administrator chooses to use reserved IP addresses, the designated client devices still go through the standard DHCP lease generation process. However, the DHCP server matches the system's MAC address to an IP address configuration, which ensures that any device with that physical MAC address receives the designated IP address settings.
An IP address reservation enables administrators to define specific settings for a particular system. While the most common setting is an unchanging IP address, other common alternatives are custom name resolution or advanced DHCP options.
These options let administrators provide custom settings to client devices with specific or unique requirements without the tedium of manual configurations.
Examples of IP address reservation settings are the following:
- Router. Designates the gateway's IP address.
- DNS server. Designates one or more name resolution servers.
- Time server. Designates a Network Time Protocol time server.
- Lease duration. Designates the lease duration time.
Network administrators can add the DHCP service to most server OSes or even many router OSes. It's important to configure IP address scopes within the service along with any additional settings, such as name resolution servers and gateways. Clients lease configurations from the scope.
IP address allocation comparison
The choice of static, dynamic or dynamic reserved IP addresses depends on the goals of the network and the team that operates it. For servers, routers and other network devices, static IP addresses are the norm, and you should plan to stick with that approach. General client computers rarely require consistent IP addresses, so network administrators can leave them at their default dynamic IP address configuration settings.
Reserved IP addresses might make the most sense in some test systems, special project servers or network print devices. IoT and smart devices might also use reserved IP address settings. These systems benefit from an unchanging identity coupled with specific settings that don't apply to other network nodes.
Some administrators even use reserved IP addresses to help guarantee the identity of network devices. On a small network, network administrators might reserve an IP address for every MAC address to ensure that nodes without one of the specified physical addresses cannot lease an IP address configuration. While this isn't a common security tactic, it can be useful when combined with other layers of security.
Reserved IP addresses let troubleshooters remotely support devices more easily due to their consistent network identity.
Finally, reserved IP addresses still provide options like DNS or router settings to client devices, which means network administrators only need to update these identities on the DHCP server for client devices to receive the new settings.
IP address settings: Real-world use
Production networks nearly always use some combination of all three IP address configuration methods. Various device roles on the network work best with just one of the approaches. While reserved IP addresses are probably the smallest grouping of configuration methods, they have their place.
A general rule of which IP addressing settings to use for the type of network configuration is as follows:
- Servers and network devices. Static IP address settings.
- Client computers, tablets and phones. Dynamic IP address settings.
- Special project systems, IoT devices, smart systems and network printers. Reserved dynamic IP address settings.
One of the most fundamental but critical settings on modern systems is the IP address configuration, which includes the address itself, the related subnet mask and the default gateway. Additional settings are useful, such as the location of one or more DNS name resolution servers.
Network administrators might manually configure IP addresses on their systems or let them lease an IP address configuration from a DHCP server. The standard DHCP approach might permit computers to lease different IP addresses over time; the specialized reserved IP address option enables network administrators to define an unchanging IP address, while still realizing the other benefits of DHCP.
Network administrators are likely to use all three methods, depending on the role of the systems. Teams should reexamine their current static and dynamic IP address allocation methods and determine when it is beneficial to use reserved IP address leases. Start with any systems requiring unique or odd IP address settings.
Damon Garn owns Cogspinner Coaction and provides freelance IT writing and editing services. He has written multiple CompTIA study guides, including the Linux+, Cloud Essentials+ and Server+ guides, and contributes extensively to TechTarget Editorial, The New Stack and CompTIA Blogs.
