39. DHCP (Dynamic Host Configuration Protocol)

Purpose of DHCP

Dynamic Host Configuration Protocol (DHCP) is an essential protocol in modern networking that enables devices to automatically discover and configure their network settings without manual intervention. This is particularly useful for client devices, such as workstations and mobile devices, which typically do not have static IP configurations.

Key Points:

Automatic Configuration: DHCP allows hosts to dynamically obtain various network configuration parameters such as:
- IP Address
- Subnet Mask
- Default Gateway
Ease of Use: When connecting devices like phones or laptops to a Wi-Fi network, users do not need to manually configure these settings.
Device Configuration: While client devices benefit from DHCP, network infrastructure devices like routers and servers are usually configured statically.
Home Networks: In small home networks, the router typically acts as the DHCP server, serving local devices.
Large Networks: In larger environments, the DHCP server is often a dedicated Windows or Linux server.

Basic Functions of DHCP

Note: All IP addresses shown are the same because they originate from Jeremy’s home router, which provides all DHCP services.

Key Commands

Releasing an IP Address

To release the current IP configuration, the command used is:

ipconfig /release

Wireshark Capture: The following captures demonstrate the ipconfig /release mechanism.

Renewing an IP Address

To renew the DHCP lease, use the command:

ipconfig /renew

The renewing process involves four messages:

DHCP DISCOVER
- Message: “Are there any DHCP servers in this network? I need an IP address!”
- Ports Used: DHCP operates on reserved ports 67 (server) and 68 (client).
DHCP OFFER
- Message: “How about this IP address?”
- The offer can be sent as either a broadcast or unicast message.
- Options Included:
  - Message Type
  - Server ID
  - Lease Time
  - Subnet, etc.
DHCP REQUEST
- Message: “I want to use the IP address that was offered.”
DHCP ACK
- Message: “Okay! You may use that address.”

DHCP Renew Process Summary

DHCP Relay

In larger networks, administrators may opt for a centralized DHCP server instead of configuring each router to serve its connected LANs. However, centralized servers do not receive broadcast messages from DHCP clients. To overcome this limitation, routers can be configured as DHCP Relay Agents.