JN0-106 Networking Fundamentals Practice Question
Which THREE fields are part of an Ethernet frame header? (Select three.)
⚠ Common exam trap
Watch out — candidates often confuse the optional 802.1Q VLAN tag or the FCS trailer as part of the Ethernet frame header, when in fact the header strictly contains only the Destination MAC, Source MAC, and EtherType/Length fields.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
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Destination MAC address
The Ethernet frame header always begins with the Destination MAC address (Option A), a 6-byte field identifying the intended recipient's hardware address, which is essential for Layer 2 forwarding. The Source MAC address (Option E) is the next 6-byte field, identifying the sending NIC so replies and switch MAC-address-table entries can be built. The EtherType field (Option C) follows (or follows the optional 802.1Q tag) and is a 2-byte value such as 0x0800 for IPv4 or 0x86DD for IPv6, telling the receiver which upper-layer protocol is encapsulated. Option B (VLAN tag 802.1Q) is not part of the standard Ethernet header; it is an optional 4-byte tag inserted only on trunk links, so it is not one of the three basic header fields. Option D (FCS) is a 4-byte CRC trailer appended to the end of the frame for error detection, not a header field.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Destination MAC address
Why this is correct
The Destination MAC address is a mandatory, 6-byte field that starts the Ethernet II header. It specifies the intended receiver's physical address, whether it is a unicast, multicast, or broadcast address, and is required for the switch to make forwarding decisions. Without a destination MAC, an Ethernet frame cannot be delivered to any station on the LAN, so it must always be present.
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VLAN tag (802.1Q)
Why it's wrong here
The VLAN tag (802.1Q) is an optional 4-byte field inserted between the Source MAC address and the EtherType field only when a frame traverses a VLAN-trunking interface. It contains a Tag Protocol Identifier (TPID) of 0x8100 and a Tag Control Information (TCI) that carries the VLAN ID and priority bits. Standard untagged or access-port frames do not include this field, so it is not a mandatory header component.
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EtherType field
Why this is correct
The EtherType field is a mandatory 2-byte field in the Ethernet II header that appears immediately after the MAC addresses and before the payload. It identifies the upper-layer protocol encapsulated in the frame, such as 0x0800 for IPv4 or 0x86DD for IPv6, allowing the receiving device to demultiplex the packet correctly. Unlike an optional VLAN tag, the EtherType is always present in every Ethernet II frame because the receiving stack requires it to interpret the payload.
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Frame Check Sequence (FCS)
Why it's wrong here
The Frame Check Sequence (FCS) is a 4-byte CRC value placed at the very end of the Ethernet frame, after the payload, and thus belongs to the frame trailer rather than the header. It is calculated over the entire frame contents on transmission and verified upon reception, enabling the network adapter to discard frames corrupted by bit errors. Because it does not contain addressing or protocol information and is not located at the beginning of the frame, it cannot be considered a header field.
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Source MAC address
Why this is correct
The Source MAC address is a mandatory 6-byte field that follows the Destination MAC address in the Ethernet II header. It identifies the transmitting interface's physical address and is essential for the receiver to learn MAC-to-port mappings through the switch's MAC address table and to generate unicast replies. While the destination MAC is necessary for forwarding, the source MAC is equally required for the data-link layer to maintain bidirectional communication and is always present.
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
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