When a client wants an IP address dynamically, it sends a DHCPDISCOVER message on the local network. Since the client has no IP yet, this request is sent as a broadcast so every device on the LAN can hear it. The destination IP used for that initial DHCP request is 255.255.255.255, the local broadcast address. This allows a DHCP server on the network to see the request and respond with an offer. 0.0.0.0 is the client’s temporary source address before it has an IP, not the destination. 192.168.1.1 is just a common router address, not the broadcast target for DHCP discovery. 255.255.0.0 is a subnet broadcast address, which DHCP does not use for the initial discovery.

MAC addresses are used to identify devices on the same local network at the data-link layer. They are 48 bits long, typically shown as six hexadecimal octets, and are assigned by the hardware manufacturer and burned into the network interface (or assigned to a virtual interface by software, but they remain a hardware-like address). DHCP does not set or assign MAC addresses; it assigns IP configuration. A 64-bit address is not the standard Ethernet MAC length, though some other contexts use 64-bit identifiers. A statement calling it a purely virtual address used by software isn’t accurate, since the MAC is the hardware (or virtualized hardware) identifier for the interface. Therefore, the true description is a 48-bit hardware address assigned to a network interface.

Config-if mode is interface configuration mode, used to set options that apply to a specific network interface. In this mode you tailor settings like the IP address, description, speed, duplex, and other per-port parameters. A WAN interface is a prime example of what you configure here, since it’s a particular interface on the device and you configure its specifics for that link. Other tasks shown fit different modes: configuring VTY lines happens in line configuration mode, while changing the hostname or a banner is done in global configuration mode. So configuring a WAN interface is the task that belongs to interface configuration mode.

The IOS image is loaded from flash memory during boot. On Cisco devices, flash holds the non-volatile store of the IOS image; the boot loader copies that image from flash into RAM to run. NVRAM stores the startup configuration, not the OS image, and RAM is volatile memory used for the running system, while ROM typically contains a bootstrap or a minimal, non-default IOS for recovery, not the normal load. So flash is the typical source for the IOS image.