When packets are transmitted from one host to another across a routed segment, which two addresses are changed? (Choose two.)

A. source IP address

B. source MAC address

C. destination IP address

D. destination MAC address

Explanation:

When packets move from one LAN segment to another LAN segment across a router, the source and destination Media Access Control (MAC) addresses in the packet change.

Packets destined for a remote network must be forwarded by a router that is typically the sending host’s default gateway. The IP address of the remote host is inserted into the packet, while the MAC address of the default gateway is inserted as the Layer 2 address. This ensures that the packet is received by the default gateway. The router then examines the destination IP address, performs a route lookup, and forwards the packet toward the destination, inserting its MAC address as the source MAC address. If the next hop is another router, then the destination MAC address is replaced with the next router’s MAC address. This process is repeated by each router along the path (inserting its own MAC address as the source MAC address and inserting the MAC address of the next router interface as the destination MAC address) until the packet is received by the remote host’s default gateway. The destination gateway then replaces the destination MAC address with the host’s MAC address and forwards the packet.

In the diagram below, when the host located at the IP address 10.0.1.3 sends data to the host located at IP address 10.1.1.3, the Layer 2 and Layer 3 destination addresses will be bb.bb.bb.bb.bb.bb and 10.1.1.3, respectively. Note that the Layer 2 destination address matches the host’s default gateway and not the address of the switch or the destination host.

It is incorrect to state that the source IP address or the destination IP address change when packets transfer from one host to another across a routed segment. The Internet Protocol (IP) addresses within the packets do not change because this information is needed to route the packet, including any data returned to the sender.

Data return to the sending host is critically dependent on the destination having a default gateway configured and its router having a route back to the sender. If either is missing or configured incorrectly, a return is not possible. For example, when managing a switch remotely with Telnet, the switch cannot be located on the other side of a router from the host being used to connect if the switch does not have a gateway configured. In this case, there will no possibility of a connection being made because the switch will not have a return path to the router.

Objective:
Routing Fundamentals
Sub-Objective:
Describe the routing concepts

References:
Cisco Documentation > Internetworking Technology Handbook > Routing Basics