DHCP Configuration

Overview

This lecture explains the key configuration settings of a DHCP server, describes how IP addresses are assigned, discusses lease duration, reservations, and details the lease renewal process.

DHCP Server Configuration Settings

• DHCP servers require configuration of an IP address range (scope) and the associated subnet mask.
• The lease duration determines how long a workstation can use the same IP address.
• DHCP servers should be configured with DNS server and default gateway IPs for client assignment.
• Additional options can be added, such as VoIP gateway IP addresses.

Address Pools, Scopes, and Reservations

• DHCP assigns IPs from a pool within a defined scope, e.g., 192.168.1.0/24.
• Large contiguous address ranges are common for DHCP scopes.
• Specific addresses can be reserved for certain devices or excluded from the pool.
• Reservations tie a fixed IP address to a device's MAC address, ensuring it always receives the same IP (static DHCP assignment).
• Reservations are managed using the device's unique MAC address.

Lease Assignment and Duration

• Most devices receive dynamically assigned IPs, subject to change after the lease expires.
• The lease duration can be set to any value; a common default is 24 hours.
• After expiration, the address returns to the pool for reassignment.
• Some DHCP servers remember previous assignments and reassign the same IP if available.

Lease Renewal Process (T1 and T2 Timers)

• When assigned an IP, a lease timer starts; at 50% of the lease (T1), the device tries to renew the lease with the original server.
• If the original server is unavailable, at 87.5% of the lease (T2), the device attempts to rebind with any available DHCP server.
• Renewing or rebinding extends the lease period for the same IP address.

Key Terms & Definitions

• DHCP (Dynamic Host Configuration Protocol) — Network protocol that automatically assigns IP configuration to devices.
• Scope — A range of IP addresses managed by a DHCP server.
• Lease Duration — Time period a device retains its assigned IP address.
• Reservation — Assignment of a specific IP address to a device's MAC address.
• T1 Timer — Check-in with the DHCP server at 50% of the lease duration.
• T2 Timer — Attempt to rebind with any DHCP server at 87.5% of the lease duration.
• MAC Address — Unique hardware address of a network interface card.

Action Items / Next Steps

• Review DHCP server configuration options on your network.
• Practice creating and managing DHCP reservations and scopes.
• Read up on DHCP lease renewal and timers for further understanding.