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Network Layer: Data Plane

The per-packet machinery of Layer 3. This module starts with how an IPv4 address is read and carved into subnets, then opens the packet itself and follows one packet as a host picks a gateway and routers match the longest prefix. It finishes with the services that make addresses usable: DHCP to hand them out, NAT to share them, MTU and fragmentation to fit packets on links, and IPv6 to replace them.

What you will be able to answer​

  • How does a mask split an address into network and host bits, and how do you find the network, broadcast and usable range?
  • How do you carve one block into unequal subnets with VLSM, and when can contiguous blocks be summarized into one prefix?
  • Which IPv4 header fields matter, what does TTL protect against, and what does the protocol field say?
  • How does a host decide between delivering locally and sending to its gateway, and whose MAC is on the frame?
  • How does a router pick one next hop with longest-prefix match, and how does that differ from the routing protocols that fill the table?
  • What does DHCP hand out, how do the DORA steps and lease timers work, and what does a 169.254 address tell you?
  • How does NAT rewrite addresses and ports, why does unsolicited inbound traffic get dropped, and what breaks peer-to-peer?
  • Why do MTU and MSS differ, how does PMTUD find the path limit, and why does blocked ICMP make big transfers hang?
  • How do IPv6 addresses compress, how do SLAAC and neighbor discovery replace DHCP and ARP, and how does dual-stack carry the transition?
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