Final answer:
DHCP operates primarily in two modes: dynamic allocation, where IP addresses are assigned for a temporary period, and fixed allocation, where a specific IP is reserved for a device based on its MAC address. ARP and CIDR are not DHCP operating modes.
Step-by-step explanation:
The Dynamic Host Configuration Protocol (DHCP) operates by automatically assigning IP addresses to devices on a network. DHCP can operate in a couple of ways:
Dynamic Allocation: With dynamic allocation, DHCP assigns an IP address from a range of addresses (called a pool) to a device for a limited period of time, or until the client informs DHCP that it no longer needs the address. This is the most common mode of operation for DHCP.
Fixed Allocation: Also known as static allocation, a DHCP server assigns a specific IP address to a device based on its MAC (Media Access Control) address. This IP address is reserved for the device and does not change, much like a permanent address.
Options A (ARP) and C (CIDR) are not methods by which DHCP operates. ARP, or Address Resolution Protocol, is used for mapping an IP address to a physical machine address that is recognized in the local network. CIDR, or Classless Inter-Domain Routing, is a method for allocating IP addresses and routing that replaces the older system based on classes A, B, and C.
Therefore, the correct answers are dynamic and fixed allocation, which correspond to letters B and D respectively.