Dynamic Host Configuration Protocol (DHCP) Simulation

What is DHCP (Dynamic Host Configuration Protocol)?

Dynamic Host Configuration Protocol (DHCP) is a network management protocol operating at the Application Layer (UDP ports 67 for Server, 68 for Client). It automates the assignment of IP addresses, subnet masks, default gateways, DNS server addresses, and lease durations to end-user devices.

The 4-Step DORA Process Explained

• D - DISCOVER (Client $\rightarrow$ Server): The client broadcasts a packet (Src: 0.0.0.0, Dst: 255.255.255.255) looking for available DHCP servers on the subnet.
• O - OFFER (Server $\rightarrow$ Client): A DHCP server responds with a unicast or broadcast offer containing an available IP address, subnet mask, gateway, and lease time.
• R - REQUEST (Client $\rightarrow$ Server): The client responds asking to accept the offered IP lease, informing all servers that it selected this lease.
• A - ACKNOWLEDGE (Server $\rightarrow$ Client): The server confirms the reservation, updating its local DHCP binding table.

APIPA (Automatic Private IP Addressing)

When a client fails to receive a DHCP response, operating systems like Windows fall back to an APIPA address within the range 169.254.0.1 to 169.254.255.254. Seeing a 169.254.x.x address indicates a complete DHCP server reachability or lease failure.

DHCP Relay Agent (ip helper-address)

Because DHCP Discover messages are Layer 2/3 broadcasts, routers do not forward them across subnets by default. When the DHCP server resides on a different network, the router interface facing the client must be configured with ip helper-address [DHCP_Server_IP] to convert incoming UDP broadcasts into targeted unicast packets routed directly to the remote DHCP Server.