Many operators deploy the Dynamic Host Configuration Protocol (DHCP) to
•Support IPv4 hosts, which do not have any way to automatically calculate interface addresses
•Support naming services (DNS)
•Log and control the mapping of interface IP addresses, including controlling who can be assigned an IP address on the network
•Control the assignment of the default gateway
•Ensure the same IP address is not used in multiple places

IPv4 DHCP

Figure 3-3 illustrates the IPv4 DHCP process.

Figure 3-3 IPv4 DHCP Operation

In this figure, host A is connected to the network and needs to obtain an IPv4 address. A, the DHCP client, sends a discover message to determine whether a DHCP server is connected to the network. If no DHCP server answers, A will not be able to obtain an IPv4 address. Get the new study guide at https://www.acedexam.com/010-151-dctech-supporting-cisco-datacenter-networking-devices/

A sends this discovery message to a broadcast address, so router C and host D also receive the message. Since neither is a DHCP server, they will not respond to the message.

Most routers can be configured as DHCP servers; the two functions have been separated in this figure for clarity.

The DHCP server B will examine its local DHCP table for A’s physical address. If the server has assigned A an address in the recent past, it will have a record associating A’s physical address with an already assigned IPv4 address. If so, B will use the existing assignment. Otherwise, B will look for an unassigned address in its address pool and create an entry for A.

Server B offers this IPv4 address to A; the offer message is sent to the broadcast address. Both D and C receive this message, but the physical address of the requesting host (A in this case) is carried in the offer packet, so C and D ignore this message.

When it receives the offer message, A will probe the network using the Address Resolution Protocol (ARP), discussed in more detail in the next section, to verify the address is not in use by another host. After verifying the proposed address is unused, A will send a request message to the DHCP server, B.

The request message is also a broadcast, so C and D receive the packet. Since neither one of these devices is a DHCP server, however, they will ignore the request message.

The DHCP server will then respond with an acknowledgment sent directly (not as a broadcast) to the client, A.

This final acknowledgment message contains other information, such as the default gateway the host should use, the prefix length for this segment, a list of name (DNS) servers available on the network, and a lease time.

The lease time allows IP addresses to be reused if a host has not been active for some time—usually measured in hours or days. Just before a lease expires, the client can request an extension.

Timing out IP address assignments allows addresses to be reused once a host moves permanently to another location in the network.