DHCP is at the heart of assigning you (and everyone) their IP address. The key word in DHCP is protocol—the guiding rules and process for Internet connections for everyone, everywhere. DHCP is consistent, accurate and works the same for every computer.

Prerequisite –Dynamic Host Configuration Protocol(DHCP) is an application layer protocol which is used to provide:. Subnet Mask (Option 1 – e.g., 255.255.255.0). Router Address (Option 3 – e.g., 192.168.1.1). DNS Address (Option 6 – e.g., 8.8.8.8).

Interactive heat transfer 4.0 programs download. Vendor Class Identifier (Option 43 – e.g., ‘unifi’ = 192.168.1.9 ##where unifi = controller)DHCP is based on a client-server model and based on discovery, offer, request, and ACK.DHCP port number for server is 67 and for the client is 68. It is a Client server protocol which uses UDP services.

IP address is assigned from a pool of addresses. In DHCP, the client and the server exchange mainly 4 DHCP messages in order to make a connection, also called DORA process, but there are 8 DHCP messages in the process.

These messages are given as below:. DHCP discover message –This is a first message generated in the communication process between server and client. This message is generated by Client host in order to discover if there is any DHCP server/servers are present in a network or not. This message is broadcasted to all devices present in a network to find the DHCP server.

This message is 342 or 576 bytes longAs shown in the figure, source MAC address (client PC) is 08002B2EAF2A, destination MAC address(server) is FFFFFFFFFFFF, source IP address is 0.0.0.0(because PC has no IP address till now) and destination IP address is 255.255.255.255 (IP address used for broadcasting). As the discover message is broadcast to find out the DHCP server or servers in the network therefore broadcast IP address and MAC address is used. DHCP offer message –The server will respond to host in this message specifying the unleased IP address and other TCP configuration information. This message is broadcasted by server. Size of message is 342 bytes. If there are more than one DHCP servers present in the network then client host will accept the first DHCP OFFER message it receives.

Also a server ID is specified in the packet in order to identify the server.Now, for the offer message, source IP address is 172.16.32.12 (server’s IP address in the example), destination IP address is 255.255.255.255 (broadcast IP address),source MAC address is 00AA00123456, destination MAC address is FFFFFFFFFFFF. Here, the offer message is broadcast by the DHCP server therefore destination IP address is broadcast IP address and destination MAC address is FFFFFFFFFFFF and the source IP address is server IP address and MAC address is server MAC address.Also the server has provided the offered IP address 192.16.32.51 and lease time of 72 hours(after this time the entry of host will be erased from the server automatically). Also the client identifier is PC MAC address (08002B2EAF2A) for all the messages. DHCP request message –When a client receives a offer message, it responds by broadcasting a DHCP request message. The client will produce a gratitutous ARP in order to find if there is any other host present in the network with same IP address. Now the server will make an entry of the client host with the offered IP address and lease time.

This IP address will not be provided by server to any other host. The destination MAC address is FFFFFFFFFFFF and the destination IP address is 255.255.255.255 and the source IP address is 172.16.32.12 and the source MAC address is 00AA00123456 (server MAC address). DHCP negative acknowledgement message –Whenever a DHCP server receives a request for IP address that is invalid according to the scopes that is configured with, it send DHCP Nak message to client.