DHCP (Dynamic Host Configuration Protocol)

DHCP automatically assigns clients an IP address, subnet mask and other configuration parameters within a local network. This removes the need to configure every host manually. DHCP uses UDP ports 67 (server) and 68 (client).

The DORA Process

A client obtains its configuration in four steps, remembered as DORA:

1. Discover – The client broadcasts a DHCP Discover into the local network to find a server.
2. Offer – A DHCP server answers with a DHCP Offer containing an available address and configuration parameters.
3. Request – The client requests the offered address with a DHCP Request.
4. Acknowledge – If the address is still available, the server confirms it with a DHCP Ack.

Key Concepts

• Lease – An address is handed out for a limited time (the lease). This prevents addresses from staying bound to a client forever and allows them to be reused.
• Scope – The range of IP addresses a server may hand out (including subnet mask).
• Options – Additional parameters distributed alongside the address, e.g. default gateway, DNS server, subnet mask.
• Reservation – A fixed IP address permanently tied to a specific MAC address, so a client always receives the same address.

Relay Agent

Because routers do not forward broadcasts, a DHCP server normally serves only its own subnet. A DHCP relay agent forwards DHCP requests from another subnet to the DHCP server (as unicast), allowing one server to serve multiple subnets.

Failover

For high availability, two DHCP servers can share the same scopes and replicate their lease information. There are two modes:

• Load balancing – Both servers hand out addresses simultaneously (default ratio 50/50, adjustable).
• Hot standby – A primary server hands out all addresses; a secondary server only takes over if the primary fails.

Failover supports a maximum of two servers and only works for IPv4 scopes.

DHCPv6 vs. SLAAC

For IPv6, address configuration does not strictly require DHCP:

• SLAAC (Stateless Address Autoconfiguration) – The host builds its own address from a global prefix advertised by the router (Router Advertisement). No central server is involved.
• DHCPv6 (stateful) – A DHCPv6 server assigns and tracks the full configuration centrally, similar to IPv4. The router still sends Router Advertisements with the managed flag so the host knows to use DHCPv6.

Useful Commands (Client)

Command	Purpose
ipconfig /all	Show the full IP configuration (adapter, MAC, IP, DNS, gateway)
ipconfig /release	Release the current address (lease)
ipconfig /renew	Request a new lease from the DHCP server