Networking › Module 6 › Lesson 3
DHCP — Automatic Addressing
How devices get IP, gateway, and DNS settings automatically—and what can go wrong
Opening
Plug in and you're online
Most devices never need a static IP. DHCP (Dynamic Host Configuration Protocol) hands out an address, subnet mask, default gateway, and DNS servers. When DHCP fails—or a rogue server answers—users "can't get on Wi‑Fi" even though the cable looks fine.
1. DORA — The Four Steps
Remember DORA: 1. Discover — Client broadcasts looking for a DHCP server. 2. Offer — Server offers an IP and options (gateway, DNS, lease time). 3. Request — Client asks to use that offer (or a preferred address). 4. Acknowledge — Server confirms the lease; the client configures its interface. After Ack, the host can ARP its gateway and start talking on the LAN.
2. Leases and Renewal
A lease is temporary. Typical home leases last hours to days. Midway through the lease, the client renews with the same server so the address stays stable while in use. When the lease expires without renewal, the address returns to the pool—another device may get it later.
3. What DHCP Hands Out
IP + mask
Your address and how big the subnet is (e.g. 192.168.1.50/24).
Default gateway
The router IP for traffic leaving the LAN—critical for internet access.
DNS servers
Where name lookups go. Wrong DNS = browsing fails even if ping by IP works.
4. Rogue DHCP Risk (Briefly)
Any device that answers Discover faster can become the "DHCP server." A rogue box can push a bad gateway or attacker-controlled DNS—steering victims into a trap. On managed networks: DHCP snooping, authorized servers only, and alerts for unexpected DHCP traffic. At home: know which device is supposed to run DHCP (usually the router) and investigate if settings suddenly change.
Knowledge Check
In the DHCP DORA sequence, the first step is:
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Knowledge Check
Besides an IP address, DHCP commonly provides:
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Knowledge Check
A rogue DHCP server is dangerous because it can:
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