Type/Length Field - Ethernet Header

Definition

• The Type/Length field is a 2-byte (16-bit) field that tells the receiver what the payload contains or how long it is, depending on the Ethernet standard being used.
• In modern Ethernet II frames, it is almost always used as the Type (EtherType) field. For example:
• 0x0800 = IPv4
• 0x86DD = IPv6
• 0x0806 = ARP

What Problem It Solves

An Ethernet frame can carry different protocols. The Type field tells the receiving device which protocol should process the data next.

What Happens If Not Used

The receiver would receive the frame but would not know whether the payload is an IPv4 packet, an IPv6 packet, an ARP message, or something else.

Easy Wording

The Type field is the label that tells the computer what kind of data is inside the Ethernet package.

Layman Example

A shipping box has a label saying "Books," "Electronics," or "Clothes." The warehouse knows how to handle the box based on that label.

Technical Example

• Your browser requests a website.
• The NIC creates an Ethernet frame.
• Destination MAC = router.
• Source MAC = your PC.
• Type = 0x0800, meaning the payload is an IPv4 packet.
• The switch forwards the frame using MAC addresses.
• The router receives the frame, reads Type = IPv4, removes the Ethernet header, and passes the payload to the IPv4 layer for routing.

Limitation: The Type field identifies what the payload is, but it cannot detect whether the frame was damaged during transmission. Frame Check Sequence (FCS), solves this by checking for transmission errors.