DHCP

The Dynamic Host Configuration Protocol (DHCP) is an application-layer protocol that automatically assigns IP addresses and other network configuration parameters to devices joining a network. It runs over UDP and sits at the application layer of the OSI model.

Without DHCP, every host on a network would need to be configured manually with a unique IP address, a subnet mask, a default gateway, and the addresses of one or more DNS servers. On any network larger than a handful of fixed hosts, manual configuration is error-prone and hard to keep consistent. DHCP centralizes that configuration in a server, handing parameters out to clients as they join.

The DORA exchange

A client obtains a lease through a four-message exchange known as DORA:

  1. Discover. The client broadcasts a DHCPDISCOVER message to find available servers. Because it does not yet have an IP address, the message is sent from 0.0.0.0 to the broadcast address 255.255.255.255 over UDP ports 67 and 68.
  2. Offer. Each server that receives the discover responds with a DHCPOFFER, proposing an IP address and lease terms.
  3. Request. The client broadcasts a DHCPREQUEST identifying the server it has chosen. The broadcast lets the other servers know their offers were declined so they can return the proposed addresses to their pools.
  4. Acknowledge. The chosen server sends a DHCPACK confirming the lease and supplying the remaining configuration, such as subnet mask, default gateway, DNS servers, lease duration, and any other options the server is configured to hand out.

Leases and renewal

A DHCP lease is temporary. The server lends an address for a fixed period rather than assigning it permanently. Before the lease expires, the client requests a renewal; if the server agrees, the lease is extended. If the client leaves the network or lets the lease lapse, the address returns to the server’s pool and can be reassigned.

Leasing rather than assigning is what lets DHCP serve far more clients than a network has addresses. A home router with a /24 subnet, expressed in CIDR notation, has only 254 usable addresses, but the devices coming and going on a guest Wi-Fi network can number in the thousands over a week. Only the currently connected hosts hold a lease at any one time.

Static and dynamic assignment

DHCP servers distinguish two kinds of assignment.

  • Dynamic. The client receives an address drawn from a pool, and the address may differ each time the client connects.
  • Static (reserved). The server maps a client’s MAC address to a fixed IP, so the same device always receives the same address. This is useful for servers, printers, and other hosts that other devices reference by IP.

Static reservation keeps the convenience of centralized configuration while giving specific hosts a stable address.

IPv4 and IPv6

DHCP was originally defined for IPv4 in RFC 2131. DHCPv6, defined in RFC 3315, serves the same purpose for IPv6 but is not the only option. IPv6 also supports Stateless Address Autoconfiguration (SLAAC), in which hosts derive addresses from router advertisements without a DHCP server. The two mechanisms coexist, and which one a network uses depends on how it is configured.

See also