At the data link layer, Ethernet encapsulation adds a frame header that uses the device’s physical (MAC) addresses to deliver the frame on the local network. The Ethernet header includes the destination MAC address and the source MAC address, along with an EtherType field that indicates the next protocol in the stack. These MAC addresses are hardware identifiers tied to network interfaces, which is why they’re called physical addresses. IP addresses belong to the network layer and are carried inside the payload after encapsulation, not added at this layer. Port numbers belong to the transport layer (TCP/UDP) and are part of the transport header, not the Ethernet data-link header.

Modulation. In the physical layer, information is carried by altering a carrier wave to encode the data. This change can be in amplitude, frequency, or phase, producing a modulated signal that travels over the medium. Modulation lets the signal fit the channel’s bandwidth and distance constraints and helps distinguish transmissions from noise. The other terms point to standards bodies or the transmission medium rather than the signaling method used to encode data.

DNS translates human-friendly hostnames into the numeric IP addresses used by computers on the Internet. When you type a web address, your device asks a DNS resolver to find the corresponding IP. The resolver may query a chain that starts at the root servers, moves to the appropriate top-level domain servers, and finally reaches the domain’s authoritative server to obtain the IP, with results cached to speed future lookups. This system makes it possible to reach sites by name rather than remembering numeric addresses, and its hierarchical design plus caching keeps lookups scalable and efficient. Other options don’t fit because DHCP assigns IP addresses and other network configuration to hosts, not domain name mappings; ARP resolves IP addresses to MAC addresses on a local network, not Internet-wide hostnames to IPs; and FTP is a file transfer protocol, not a name-to-address resolution service.

The main concept is error detection using a Frame Check Sequence (CRC). The FCS is a CRC value appended to the end of the frame. The sender computes the CRC over the frame’s contents (header and data) and places the result in the FCS. When the frame is received, the NIC recalculates the CRC over what was received and compares it to the FCS. If they match, the frame is considered to have arrived without detectable errors; if they don’t, a transmission error occurred and the frame is discarded or flagged for error handling. This mechanism detects many bit errors, including burst errors, but it does not validate MAC addresses or logical addresses, and it applies to the whole frame rather than just the header.