Computer Networking · Intermediate level

Intermediate level · Module 2: DHCP, DNS and NAT · Lesson 1 of 3

DHCP in Depth

Configure DHCP so devices get the right settings automatically.

In this lesson you will

  • Describe the DHCP exchange
  • Configure scopes, reservations and options

Key concepts

discover, offer, request, acknowledgescopereservationlease

The lesson

A device gets an address in four steps: Discover, Offer, Request and Acknowledge (DORA).

A scope defines the range, mask, gateway, DNS servers and lease time. Reservations give printers and servers the same address every time.

Keep static addresses outside the DHCP range to avoid conflicts. On networks with several VLANs, a DHCP relay forwards requests to the server.

Step by step

  1. Configure a scope.
  2. Add a reservation for a printer.
  3. Capture or observe a DHCP exchange.
  4. Fix a planted DHCP fault.
Workflow

How a laptop gets its address: DHCP step by step

The four messages between a new device and the DHCP server.

  1. Discover

    The laptop, with no address yet, broadcasts 'is there a DHCP server?'

    client

  2. Offer

    The server offers an unused address from its scope, with mask, gateway, DNS and lease time.

    DHCP server

  3. Request

    The laptop broadcasts that it accepts this offer (so other servers know).

    client

  4. Acknowledge

    The server confirms and records the lease; the laptop configures itself.

    DHCP server

Halfway through the lease the laptop asks to renew it, so it usually keeps the same address.

Why it works

Discover and Request are broadcasts, which routers do not forward. With several VLANs, a DHCP relay on the router forwards them to the server. If no server answers, Windows assigns a 169.254 address.

Practice activity

Configure DHCP for the lab network.

Expected result: Working DHCP with reservations and documented scope.

Check your understanding

  1. Name the four DHCP steps.

    Show answer

    Discover, Offer, Request, Acknowledge.

  2. Why use a reservation for a printer?

    Show answer

    So it always has the same address.

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