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DHCP (Dynamic Host Configuration Protocol)

Plug into Wi-Fi and seconds later you have an IP — that's DHCP doing the work.

DHCPIPNetwork Layer
核心 · Key Idea

In one line: DHCP lets a freshly-connected device automatically receive an IP address, subnet mask, gateway, and DNS without manual config. It runs over UDP, ports 67 (server) / 68 (client).

What it is#

The classic four-step handshake — DORA:

1. Discover (broadcast) : client → "any DHCP servers around?"
2. Offer    (uni/bcast) : server → "you can have 192.168.1.50 + gateway + DNS + 24h lease"
3. Request  (broadcast) : client → "I accept X's offer" (picks one when multiple servers)
4. ACK      (uni/bcast) : server → "confirmed"

After that the client may use the IP. Halfway through the lease, the client unicasts a renew to the same server.

Analogy#

打个比方 · Analogy

DHCP = the hotel front desk: present your ID (MAC) → desk hands you a room number (IP) + key code + floor map (subnet mask + gateway + DNS) + check-out time (lease). Renew before check-out.

Key concepts#

LeaseLease
How long the IP is loaned; expires unless renewed. Home routers default to ~24h; enterprise may use 1h.
ReservationReservation
Bind a MAC to a specific IP — common for servers / printers.
Scope / PoolScope / Pool
Range the DHCP server can hand out, e.g. 192.168.1.100-200.
DHCP RelayDHCP Relay
Because DHCP is broadcast, routers must forward Discover packets to a central DHCP server across subnets.
DHCPv6 / SLAACIPv6 auto-config
IPv6 supports DHCPv6 or SLAAC — router advertises a prefix and hosts derive their own IP.
Option 43 / 60 / 66DHCP Options
Deliver extras: PXE boot, TFTP server, domain, NTP, etc.

How it works#

Across subnets, routers run DHCP relay (ip helper-address) to unicast broadcasts to the central server.

Practical notes#

  • ip a / ipconfig /all — check if the IP came from DHCP (look for the DHCP server field).
  • Renew / re-request: Linux dhclient -r && dhclient; Windows ipconfig /release && /renew; macOS "Renew DHCP Lease".
  • 169.254.x.x (APIPA) — fallback link-local address. Seeing this means DHCP failed.
  • Servers: home routers ship a built-in one; enterprise uses ISC DHCP / Kea / Windows Server / RouterOS.
  • PXE network boot: DHCP option 66/67 tells clients the TFTP server and boot file.
  • Security: DHCP Snooping on switches identifies legitimate DHCP servers and blocks rogue ones.
  • Static vs DHCP: prefer reservations for servers / printers / monitoring — stable yet centrally manageable.

Easy confusions#

DHCP
**Receive IP config** (address, gateway, DNS).
The step before you go online.
DNS
**Resolve domain → IP**.
Used on every connection.

Further reading#