DHCP: how devices get an IP address automatically

The problem DHCP solves

Every device needs an IP address, subnet mask, default gateway and DNS server. Typing these by hand on hundreds of devices would be slow and full of mistakes. DHCP (Dynamic Host Configuration Protocol) does it automatically.

The four steps: "DORA"

1. Discover: the new device shouts to the network, "Is there a DHCP server here?"
2. Offer: the DHCP server replies, "You can use this address."
3. Request: the device says, "Yes please, I'll take that address."
4. Acknowledge: the server confirms, and the device starts using the settings.

Leases

Addresses are loaned, not given forever. Each one has a lease time (for example 8 hours or 1 day). Devices renew before the lease ends; addresses from devices that leave go back into the pool.

Where the DHCP server lives

• At home: inside your Wi-Fi router.
• In offices: often a server or the core switch, or a firewall.

When it fails

If a device shows a 169.254.x.x address, it couldn't reach a DHCP server. Check the cable or Wi-Fi, the VLAN, and whether the DHCP pool has run out of addresses. To request a new address:

ipconfig /release
ipconfig /renew

Security tip: in business networks, DHCP snooping on switches blocks fake DHCP servers. See the switch hardening playbook.

Real-world example: a busy Saturday at a café

A small café offers free Wi-Fi. Its router runs DHCP with a pool of, say, 50 addresses. On a normal weekday that is plenty: customers come and go, and addresses from phones that have left go back into the pool when their lease ends.

On a busy Saturday, the café is full, and every customer's phone, tablet and watch asks for an address. If the lease time is long (a day, for example), phones that left in the morning still "hold" their room keys. By the afternoon the pool is empty, and new customers see Wi-Fi connected but nothing loads, with a 169.254.x.x address on their device.

The usual fixes are a larger pool or a shorter lease for the guest network, such as an hour or two, so addresses return quickly. It is exactly like a hotel shortening its stays during a busy festival.

Try this: on Windows, run ipconfig /all and find the lines "Lease Obtained" and "Lease Expires". They tell you when your device checked in and when its key runs out.