What is the Source Address Field (within an Ethernet Frame)?

This blog post briefly defines the Source Address field, within the Ethernet Frame.

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The Source Address field is a 6-byte (48-bit) field that occurs just after the Destination Address within the Ethernet frame.

I show an illustration of the IEEE 802.3 (Basic) Ethernet frame, with the Source Address field highlighted.

IEEE 802.3 Basic Ethernet Frame with the Source Address Field Highlighted

Figure 1, Illustration of the IEEE 802.3 (Basic) Ethernet frame, with the Source Address field highlighted

Comparison to the Destination Address Field

When I was discussing the Destination Address field, I mentioned that the Destination Address field contains the address location, or the Ethernet station/port that we are sending the Ethernet frame to.

In contrast, the Source Address field contains the address of the Ethernet station, that is generating/transmitting the Ethernet frame.

Further, where the Destination Address field can contain either a Unicast (or Physical) Port Address, a Multicast, or the Broadcast Address; the Source Address field ONLY contains the Unicast (or Physical) Port Address.

In Figure 2, I show an Ethernet station (with MAC Address 0xFE005E276C7D) sending an Ethernet frame another Ethernet station (within MAC Address 0xFE005F376DEF). I also show (how the Destination and Source Address fields, within the frame, relates to the Ethernet station addresses.

Ethernet Station A transmitting an Ethernet Frame to Ethernet Station B - Source and Destination Address fields shown

Figure 2, Ethernet station A (with MAC Address =0xFE005E276C7D) sends an Ethernet frame to Ethernet Station B (with MAC Address = 0xFE005F376DEF).

Ethernet Hardware Processing of the Source Address Field

In most cases, Ethernet switching and routing circuitry does not pay too much attention the Source Address field, within each Ethernet frame. This hardware is almost always more focused on the value of the Destination Address (and making sure the Ethernet frames are properly switched throughout the network).

However, there are cases where the Ethernet Circuitry will use and evaluate the Source Address field. Some of these cases include troubleshooting problems. Additionally, the Ethernet equipment will pay attention to the Source Address field, whenever a new router (or switch) powers up and it needs to build up a routing table (e.g., a table that associates switch ports with Ethernet addresses).

In another blog post, I discuss the Address Resolution Protocol, in which the Source Address field comes in handy.

Nonethless, the Source Address field does not typically undergo near the evaluation and processing as does the Destination Address.

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Author: Darrell Smith

Darrell Smith has more than 30 years of experience as an Electrical Engineer. He has about 20 years of experience as an Applications Engineer and the remainder of his time was spent in Hardware Design and Product Marketing. He will now be sharing his wealth of knowledge on this blog.

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