As an IP packet is routed across subnets, which value gets updated at each hop?

In network communications, when an IP packet traverses multiple subnets to reach its destination, certain header information is modified at each hop to facilitate proper routing and delivery.

Destination MAC Address:

Role: The Media Access Control (MAC) address is a hardware identifier used at the Data Link layer (Layer 2) of the OSI model.

Update Process:

As a packet moves from one router to the next, the destination MAC address is updated to reflect the MAC address of the next-hop device.

This ensures that the packet is correctly forwarded at the Data Link layer within each local network segment.

Explanation:

When a router forwards a packet, it encapsulates the IP packet within a new Ethernet frame.

The router sets the destination MAC address in this frame to the MAC address of the next-hop router or the final destination device if it is on the same local network.

This process repeats at each hop, with each router updating the destination MAC address to ensure proper Layer 2 delivery.

Other Header Fields:

Destination IP Address:

Remains unchanged throughout the packet's journey, ensuring end-to-end delivery to the correct device.

Source IP Address:

Also remains unchanged, identifying the original sender of the packet.

Subnet Mask:

Not a field within the packet header; it is used by routers to determine the network portion of an IP address but is not transmitted with the packet.

References:

For a detailed understanding of how routers handle packet forwarding and the role of MAC addresses in this process, refer to the RUCKUS ICX Switch Configuration Guide, which provides insights into Layer 2 and Layer 3 forwarding mechanisms.