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DHCP Server Settings Overview

Jun 16, 2025

Overview

This lecture explains key DHCP (Dynamic Host Configuration Protocol) server settings, including address pools, lease times, reservations, options, and the process by which DHCP allocates and manages IP addresses for network devices.

DHCP Scopes and Address Pools

  • A DHCP scope defines the range (pool) of IP addresses assigned to devices on a specific subnet.
  • Scopes also specify excluded addresses that should not be handed out by DHCP.
  • Each scope includes subnet mask details, lease durations, and additional configuration options.

Lease Duration and Renewal Process

  • IP address leases are temporary, with durations set by the DHCP server.
  • Devices must renew their lease before expiration or risk losing the IP address.
  • T1 timer (50% lease time) triggers a renewal attempt to the original DHCP server.
  • T2 timer (87.5% lease time) allows renewal attempts to any DHCP server if the original is unavailable.
  • If a device disconnects or powers off, its IP can be released for reuse.

DHCP Reservations

  • Address reservations tie a specific device's MAC address to a fixed IP address, ensuring consistency.
  • Managed centrally in the DHCP server, making changes easier than manual device configuration.
  • Reservations are also called static DHCP assignments or IP reservations.

DHCP Options

  • DHCP options allow configuration of additional TCP/IP settings (e.g., default gateway, DNS servers).
  • There are 254 numbered DHCP options, such as option 129 for Voiceover IP servers or 135 for HTTP proxies.
  • Not all DHCP servers support every option; server compatibility must be checked.

DHCP Management and Configuration

  • All DHCP configurations (address pool, exclusions, lease time, options, reservations) are managed via the DHCP server interface.
  • Changes to network settings can be done centrally on the DHCP server rather than individual devices.

Key Terms & Definitions

  • DHCP (Dynamic Host Configuration Protocol) — Automatically assigns IP addresses and network configuration to devices.
  • Scope — Defines a range of IP addresses and settings available for assignment.
  • Lease — Temporary assignment of an IP address for a set period.
  • Reservation — Assignment ensuring a device always receives the same IP based on MAC address.
  • DHCP Options — Additional settings passed to devices, such as DNS and gateway info.
  • T1/T2 Timers — Timers for lease renewal: T1 (50%), T2 (87.5% of lease duration).

Action Items / Next Steps

  • Review how to configure DHCP scopes, reservations, and options in your lab environment.
  • Check which DHCP options are supported by your network's DHCP server.
  • Prepare for potential exam questions covering lease times, DHCP options, and the renewal process.

Full transcript