DHCP (Dynamic Host Configuration Protocol)

• Automatically assigns IP addresses and network configuration to devices on a network to eliminate manual configuration
• Operates on client-server model where DHCP server maintains pool of available IP addresses and leases them to requesting clients
• Uses UDP ports 67 (server) and 68 (client) for communication
• Prevents IP address conflicts by tracking which addresses are assigned and for how long

DHCP Process (DORA)

• Discover: Client broadcasts DHCP Discover message to find available DHCP servers
• Offer: Server responds with DHCP Offer containing available IP address and configuration parameters
• Request: Client broadcasts DHCP Request to accept specific server’s offer
• Acknowledge: Server sends DHCP ACK confirming lease and finalizing configuration

Key Configuration Parameters

• IP Address: Primary address assigned to client from configured pool
• Subnet Mask: Defines network portion of IP address
• Default Gateway: Router IP for reaching other networks
• DNS Servers: Domain name resolution servers (primary and secondary)
• Lease Duration: Time period client can use assigned IP (typically 24 hours to 8 days)
• Domain Name: Local domain suffix for name resolution

DHCP Relay Agent

• Forwards DHCP requests across subnets since DHCP uses broadcast traffic (doesn’t cross routers by default)
• Router interface configured with ip helper-address <DHCP-server-IP> command
• Converts broadcast to unicast when forwarding to DHCP server
• Essential for centralized DHCP server serving multiple VLANs/subnets

Vocabulary

• Lease: Time-bound assignment of IP address to client device
• Scope: Range of IP addresses available for DHCP assignment on specific subnet
• Binding: Association between client MAC address and assigned IP address
• Helper Address: Router configuration directing DHCP broadcasts to specific server

Notes

• DHCP servers should have static IP addresses to ensure consistent availability
• Always exclude static device IPs from DHCP pools to prevent conflicts
• Multiple DHCP servers can serve same subnet for redundancy, but requires careful coordination
• Client retries lease renewal at 50% of lease duration, then broadcasts at 87.5% if original server unavailable