LANs: correcting the model.

A local area network connects devices within a limited area and gives them a shared way to communicate. The common star topology connects each device to a central switch or router. It is scalable and easier to troubleshoot than a bus or ring, although the central device becomes an important point of failure.

Addresses need structure

A subnet mask separates the network portion of an IPv4 address from the host portion. I initially wrote masks using 225; the correct octet is 255. That small typo revealed that I was remembering the shape of the number instead of understanding the bit boundary it represented.

DHCP automates address configuration through Discover, Offer, Request, and Acknowledge. The client and server use UDP ports 68 and 67. Reservations are useful for systems—such as a DNS server or Raspberry Pi—that should keep a predictable address without manually configuring every network setting.

ARP stays inside the local network

My first explanation incorrectly treated ARP as the mechanism that reaches an external destination. ARP actually resolves a local IPv4 address to a local MAC address. If the destination is on the same subnet, the sender asks which device owns that address and caches the reply.

For an external destination, the computer does not ask for the remote server’s MAC address. It recognizes that the IP is outside its subnet, uses ARP to learn the default gateway’s MAC address, and sends the Ethernet frame to the router. The router then handles the next hop.

Why the correction mattered

The useful part of this lesson was not memorizing four networking terms. It was finding the exact point where my mental model failed. Recalling the process without notes made the mistake visible; tracing one local and one external destination fixed it.