DHCP Reservation or Static IP Address

Introduction

Every network device needs to have an Internet Protocol (IP) address to be able to communicate with other devices on the local network or over the internet. There are two ways for a device to get an IP address; they can be automatically be assigned one from a Dynamic Host Control Protocol (DHCP) server or they can be manually configured with one directly on the Network Interface Card (NIC) of the device.

By default, when a device connects to a network, they are assigned a random IP address from a pre-allocated pool of addresses specified on the DHCP server. For most devices having a random and changing IP address is fine but there are a few use cases where it is better to keep giving the same device the same IP address. If clients are needing to connect to a network device then in most cases it is best for the “server” device to have a consistent IP address.

There are two types of network devices; clients and servers.

• Clients devices are devices that are used to connect to server devices.
• Server devices are devices that provide some kind of service to client devices.

A device can be classified as both a client device and a server device.

There are two different ways to make sure that a device keeps the same IP address on the network. The device can be manually configured with the static network settings or a reservation can be set up on the DHCP server so that every time the device checks in and asks for the DHCP server for a new IP address, the server checks the Media Access Control (MAC) address of the requesting device and if it matches a device in the reservation list, it provides the device with the reserved IP address.

Advantages of using DHCP reservation

• The IP addresses can be managed easily across the network as you can see exactly what addresses are in use by looking at the DHCP reservation table.
• If you update your IP address range of the local network, change the subnet size/mask, the default gateway, or the DNS server, you can update the values on the DHCP server and all you need to do is reboot the devices and they will get the latest configuration changes.
• Administrators might not always have the correct access to change 3rd party vendor devices, such as managed phone systems, so they can control what IP address the device gets from the DHCP server.
• There are over 100 different DHCP Options settings that can be configured to easily provide custom parameters to devices.

Disadvantages of using DHCP reservation

• Not all network devices are DHCP aware and can’t be provided with an address from the DHCP server so must be manually configured.
• There is a chance for rogue DHCP servers to be set up on the network.
• If there is an issue with the DHCP server and it stops providing addresses the devices on the network will stop working once the address they have been provided expires.

When you should not use DHCP reservation

DHCP reservation should be avoided for critical infrastructure devices such as firewalls, routers, switches, and domain controllers, as if there is a network outage you will likely need to access these devices and if they lose their DHCP address you will struggle to get access quickly.

Conclusion

Personally, unless a device requires a fixed IP address and always needs to be accessible even when there is an issue with the DHCP server, I always use DHCP reservation and leave the device configured to automatically receive an address from the DHCP server.