When a packet crosses routers, some fields stay the same end to end, while others change at each link. That distinction shows up constantly in CCNA study because it ties together IPv4 addressing, default gateways, ARP, and routing decisions. If you want more mixed-topic review after this exercise, use the relevant questions in the public bank of CCNA practice questions and filter toward IP Connectivity and Network Fundamentals.
Study example: trace one packet across two subnets
Study example addressing (ordinary Ethernet forwarding, with no NAT, tunneling or policy that rewrites endpoint addresses):
PC1: 192.168.10.50/24, default gateway 192.168.10.1 R1 G0/0: 192.168.10.1/24 R1 G0/1: 192.168.20.1/24 PC2: 192.168.20.80/24, default gateway 192.168.20.1
PC1 sends a packet to PC2. Because 192.168.20.80 is not in PC1’s local /24 network, PC1 sends the frame to its default gateway, R1 G0/0.
On the first link, the Layer 3 packet has source IP 192.168.10.50 and destination IP 192.168.20.80. Those IP addresses do not change just because the packet is moving toward another subnet. But the Layer 2 frame must match the local link. So PC1 builds an Ethernet frame with source MAC = PC1’s NIC and destination MAC = R1 G0/0.
R1 receives the frame, removes the Layer 2 header, checks the destination IP, and sees that 192.168.20.0/24 is directly connected on G0/1. R1 then creates a new Ethernet frame for the second link. The Layer 3 source IP is still 192.168.10.50, and the Layer 3 destination IP is still 192.168.20.80. The router also decrements IPv4 TTL and updates the header checksum. The replacement Layer 2 information is: source MAC becomes R1 G0/1, and destination MAC becomes PC2’s NIC.
So the quick rule is simple: routers rewrite the frame header for each new link, but they do not normally rewrite the source and destination IPv4 addresses during ordinary routing.
Multiple-choice question with explained answer
A packet travels from PC1 to PC2 using the addressing above. Which statement is correct when R1 forwards the packet out G0/1?
- A: The source IP changes to 192.168.20.1, and the destination IP stays 192.168.20.80.
- B: The source MAC changes to R1 G0/1, and the destination MAC changes to PC2’s MAC.
- C: The source IP changes to 192.168.10.1, and the destination MAC stays R1 G0/0’s MAC.
- D: The destination IP changes to 192.168.20.1, and the source MAC stays PC1’s MAC.
Correct answer: B.
Why B is right: once R1 forwards onto the second subnet, it builds a new frame for that outgoing interface. That means the source MAC is now R1 G0/1, and the destination MAC is PC2’s MAC.
Why the others are wrong: A and C incorrectly change the IP source. Ordinary routing does not replace the original sender’s IP address. D incorrectly changes the destination IP to the gateway and incorrectly keeps the old source MAC from the previous link.
What to check when you get stuck
If a routing trace question feels confusing, write down three things separately: the sender’s IP packet, the current link, and the next-hop device on that link. That prevents mixing Layer 2 and Layer 3 facts. For broader review, a readable CCNA study guide can help you connect this exercise to ARP, directly connected routes, and default gateway logic. The fastest self-check is this: different subnet means send to the gateway’s MAC on the local link, but keep the final destination IP unchanged.
Official exam reference: Cisco CCNA exam topics. These study examples are independently written.