📘 03-04: DHCP¶

❓ What Is DHCP?¶

DHCP (Dynamic Host Configuration Protocol) automatically leases IP configuration to clients, eliminating manual (static) setup on each host.

It provides: - IP address - Subnet mask - Default gateway - DNS server(s) - (Optionally) NTP, domain name, and more

Ports: server UDP 67, client UDP 68.

📌 The DORA Process¶

1. DISCOVER  — client broadcasts "any DHCP servers?"
2. OFFER     — server offers an available address
3. REQUEST   — client requests the offered address
4. ACK       — server confirms the lease

Mnemonic: D-O-R-A.

📌 Scope Concepts¶

Term	Meaning
Scope	The range of addresses a server can hand out
Reservation	Binds a specific MAC to always get the same IP
Exclusion	Addresses kept out of the pool (for static devices)
Lease time	How long a client keeps its address

🧮 DHCP Across Subnets: Relay¶

DHCP Discover is a broadcast confined to its VLAN/subnet. To use a central DHCP server on another subnet, configure a DHCP relay (ip helper-address) on the router/L3 switch to forward requests as unicast.

📡 When DHCP Fails¶

A host that can't reach a DHCP server self-assigns an APIPA address (169.254.x.x) — a telltale sign of a DHCP/connectivity problem.

Tip

Key idea — DHCP automates IP config via DORA (ports 67/68). Use reservations for consistent device addresses and a DHCP relay to serve clients across subnets. APIPA (169.254.x.x) = DHCP failed.

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