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Tuesday, February 26, 2019

What Is DHCP (Dynamic Host Configuration Protocol)?

IP4 Dialog with Automatic settings selected

The Dynamic Host Configuration Protocol (DHCP) is integral to networks and controls what IP addresses devices receive so they can communicate with the internet. Usually, IP assignment is automated, but if you need static IPs, familiarity with DHCP is essential.

DHCP Can Handle IP Assignments

Every device that connects to a network needs an IP address. In the early days of networking, users manually assigned themselves an IP address, but that’s a cumbersome task, especially for places with many devices, such as a corporate office. DHCP, in part, automates this process, which makes connecting devices to the network far easier. DHCP servers or routers handle this process based on a set of defined rules. Most routers are set to use a 192.168.0.x range, for instance, so you’ll commonly see IP addresses like this in home networks.

The process is pretty straight forward. When a client (a computer, IOT device, tablet, cell phone, etc.) connects to the network, it sends out a signal (called DHCPDISCOVER) to the DHCP server (or router). The server responds with all the rules and settings for the network and an IP address for use (a DHCPOFFER). The client acknowledges the information and asks permission to use the assigned address (a DHCPREQUEST message). Finally, the DHCP server acknowledges the request, and the client is free to connect to the network.

DHCP Controls the Range of IP Addresses

DHCP start and end address configuration

You can configure DHCP to control the range of IP addresses available for use. If you state that range as starting at and the end as, then all available addresses will fall somewhere within that range. You’ll never see a device assigned to Also, bear in mind that the start IP ( in this example) is reserved for the router. Some routers only list a starting address and then include an option for a maximum number of users (which determines the end address).

The upside to this is you can control how many devices connect to your network simultaneously (no more than 100 in this example). But the downside is if you set the range too small you can unintentionally prevent connection of new devices. To allow for a lower range of IP addresses, DHCP servers only lease out IP addresses to devices.

Dynamically Assigned Addresses are Temporary

When a DHCP server assigns an IP Address, it does so under a lease system. The machine retains this IP address for a set number of days, after which it can try to renew the IP address. If no renewal signal is sent (such as a decommissioned machine), then the DHCP server reclaims the IP address to assign to another device. When the renewal signal is detected, the device retains its IP address for another set of days. This is why your IP address may appear to change from time to time if you use the ipconfig option often.

It’s possible for two devices to end up with the same IP, such as a VM machine that spends most of its time offline. The VM machine won’t be able to send the renew signal, so its IP address will be handed out to another machine. When the VM is brought back up, it still has a record of the old IP address (especially if restored from a snapshot), but it won’t be able to use that IP address since it is taken. Without that permission, it can’t connect to the network until a new IP is assigned. But using dynamic IP addresses should prevent this type of scenario.

Static IP Addresses are Necessary For Some Devices

Read the remaining 5 paragraphs

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