DHCP IP Address Allocation

DHCP Allocation Methods

• There are three methods of DHCP allocation: Manual, Automatic, and Dynamic allocation.
• Manual allocation is best suited for giving IP addresses to servers, as it guarantees a permanent and known location in the network.
• Dynamic allocation is the most interesting method, as it involves assigning, reclaiming, and reusing network addresses.
• It's useful in cases where there is a limited amount of network addresses, and computers temporarily connect and disconnect to the network.

Dynamic Allocation

• The client requests a network address for a limited period of time (lease).
• The DHCP server allocates an address, marks it as 'used', and notifies the client about the address and lease time.
• The client can extend its lease, ask for a permanent assignment, or release the address back to the server before the lease expires.

Renewing and Acquiring Addresses

• The client holds two times in its memory: time1 and time2.
• Time1 is when the client starts to ask its server for renewing the lease, and time2 is when the client starts to ask other servers for an address.
• If the client doesn't get an answer, it waits half of the time left before sending DHCPREQUEST again.
• The shortest waiting time is one minute.

Configuration Parameters Delivery

• The DHCP server supplies clients with configuration parameters defined in the Host Requirements RFCs.
• The server provides a permanent storage of network parameters for clients.
• The storage model is a set of key-value pairs for each client, where the key is a unique identifier and the value contains the configuration parameters.

Message Format

• The DHCP message format is based on the BOOTP message format.
• The message format includes fields such as op, htype, hlen, hops, xid, secs, flags, ciaddr, yiaddr, siaddr, giaddr, chaddr, sname, and file.

Client/Server Model

• The client and server negotiate in a series of messages to get the parameters the client needs.
• The client broadcasts a DHCPDISCOVER message, and each server responds with a DHCPOFFER message.
• The client chooses one server and broadcasts a DHCPREQUEST message, and the selected server responds with a DHCPACK message.

Steps of Communication

• The client broadcasts a DHCPDISCOVER message.
• Each server responds with a DHCPOFFER message.
• The client chooses one server and broadcasts a DHCPREQUEST message.
• The selected server responds with a DHCPACK message.
• The client receives the DHCPACK message and is configured.
• If the client receives a DHCPNAK message, it restarts the configuration process.
• The client may choose to relinquish its lease on a network address by sending a DHCPRELEASE message.
• The server receives the DHCPRELEASE message and marks the lease as free.

DHCP Fundamentals

• DHCP (Dynamic Host Configuration Protocol) is a network management protocol used to dynamically assign IP addresses to devices on a network.
• DHCP automates and centrally manages IP configurations, replacing manual assignments by network administrators.

DHCP Operations

• DHCP operations fall into four basic phases: IP lease request, IP lease offer, IP lease selection, and IP lease acknowledgement.

IP Lease Request

• A client broadcasts a request for an IP address to available DHCP servers on the local physical subnet.
• The client can request its last-known IP address, and the server may grant the request if it is still valid.

IP Lease Offer

• The DHCP server responds with a DHCPOFFER message containing the client's MAC address, IP address, subnet mask, lease duration, and server's IP address.
• The server determines the configuration based on the client's hardware address (CHADDR field).

IP Lease Selection and Acknowledgement

• The client accepts an offer by broadcasting a DHCPREQUEST message, and other DHCP servers withdraw their offers.
• The DHCP server responds with a DHCPACK packet, including the lease duration and any other requested configuration information.
• The TCP/IP configuration process is complete after the client receives the DHCPACK packet.

DHCP Message Types

• DHCPDISCOVER: client broadcast to locate available servers
• DHCPOFFER: server response to DHCPDISCOVER with configuration offer
• DHCPREQUEST: client message to servers for requesting offered parameters or confirming correctness of previously allocated address
• DHCPACK: server response to DHCPREQUEST, including lease duration and configuration information

Additional DHCP Features

• A client can reuse a previously allocated network address by including it in the 'requested IP address' option in the DHCPREQUEST message.
• A client can obtain local configuration parameters without allocating a new address using a DHCPINFORM request message.