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CCNA 200-301 v1.1 reference — Network Fundamentals

Packet walk through a network

Follow a packet hop by hop: switching inside a VLAN, routing between subnets, ARP, MAC rewrites and longest-prefix forwarding.

Objective 1.1Objective 1.13Objective 3.2v1.1 exam target
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A packet between subnets changes hands at each hop: the Layer 2 frame (source/destination MAC) is rewritten by every router, while the Layer 3 packet (source/destination IP) stays the same end to end. The host sends the frame to its default gateway's MAC, not the destination's.

Encapsulation at each hop

IP addresses are constant; MACs are rewritten per link.
HopSrc MACDst MACSrc IPDst IP
PC → R1PC MACR1 G0/0 MACPC IPServer IP
R1 → R2 linkR1 G0/1 MACR2 G0/0 MACPC IPServer IP
R2 → ServerR2 G0/1 MACServer MACPC IPServer IP

IP addresses are constant; MACs are rewritten per link.

End-to-end sequence

  1. PC computes that the destination is off-subnet (its mask says so) → needs the default gateway.
  2. PC ARPs for the gateway IP → learns gateway MAC → encapsulates frame to it.
  3. Switch floods/learns as normal → frame reaches the router interface.
  4. Router strips L2, looks up destination in its routing table (longest-prefix match), decrements TTL, re-encapsulates for the next hop.
  5. Final router finds a connected route → ARPs for the server → delivers.
Trace 10.1.1.10 → 10.2.2.20 through one router
PC
10.1.1.10/24, gw 10.1.1.1
R1
G0/0 10.1.1.1, G0/1 10.2.2.1
Server
10.2.2.20/24
  1. PC: 10.2.2.20 is outside 10.1.1.0/24 → send to gateway 10.1.1.1.
  2. PC ARPs for 10.1.1.1 (not for the server) → builds frame PC_MAC → R1G00_MAC.
  3. R1 receives, looks up 10.2.2.0/24 → connected on G0/1 → ARPs for 10.2.2.20 on that interface.
  4. R1 rewrites frame R1G01_MAC → Server_MAC, TTL decremented, delivers.

The server sees source IP 10.1.1.10 but source MAC R1-G0/1 — L3 source constant, L2 rewritten.

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Related resources

Practise this topicSource topic 3: OSI Model & TCP/IP SuitePractise topic 3 deckSource topic 10: IPv4 HeaderPractise topic 10 deckSource topic 11: Routing Fundamentals & Static RoutingPractise topic 11 deckSource topic 12: The Life of a Packet

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Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.

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Packet walk through a network | CCNA Study